mnist tutorial: fix image rendering in docs + fix toc rendering (#6311)
mnist tutorial: fix image rendering in docs + fix toc rendering (#6311)

* Misc docs improvements

* Fix image rendering issues in docs + fixes toc rendering

LinkHS/incubator-mxnet,xcgoner/dist-mxnet,weleen/mxnet,mbaijal/incubator-mxnet,sergeykolychev/mxnet,likelyzhao/mxnet,EvanzzzZ/mxnet,nicklhy/mxnet,larroy/mxnet,yajiedesign/mxnet,smolix/incubator-mxnet,solin319/incubator-mxnet,thirdwing/mxnet,precedenceguo/mxnet,rahul003/mxnet,arank/mxnet,apaleyes/mxnet,ykim362/mxnet,zhreshold/mxnet,hotpxl/mxnet,jermainewang/mxnet,hesseltuinhof/mxnet,ptrendx/mxnet,CodingCat/mxnet,DR08/mxnet,tornadomeet/mxnet,jermainewang/mxnet,tlby/mxnet,zhreshold/mxnet,LinkHS/incubator-mxnet,Northrend/mxnet,Prasad9/incubator-mxnet,vikingMei/mxnet,pluskid/mxnet,apache/incubator-mxnet,ForkedReposBak/mxnet,crazy-cat/incubator-mxnet,fullfanta/mxnet,arikpoz/mxnet,LinkHS/incubator-mxnet,arank/mxnet,navrasio/mxnet,stefanhenneking/mxnet,reminisce/mxnet,gautamkmr/incubator-mxnet,tornadomeet/mxnet,vikingMei/mxnet,jiajiechen/mxnet,luoyetx/mxnet,jennyzhang0215/incubator-mxnet,hpi-xnor/BMXNet,apaleyes/mxnet,Ldpe2G/mxnet,saurabh3949/mxnet,sxjscience/mxnet,smolix/incubator-mxnet,leezu/mxnet,piiswrong/mxnet,hpi-xnor/BMXNet,apache/incubator-mxnet,antoan2/incubator-mxnet,kkk669/mxnet,ptrendx/mxnet,sergeykolychev/mxnet,Ldpe2G/mxnet,sxjscience/mxnet,crazy-cat/incubator-mxnet,EvanzzzZ/mxnet,pluskid/mxnet,nicklhy/mxnet,lxn2/mxnet,DR08/mxnet,ShownX/incubator-mxnet,tornadomeet/mxnet,vikingMei/mxnet,arank/mxnet,eric-haibin-lin/mxnet,ShownX/incubator-mxnet,sergeykolychev/mxnet,gautamkmr/incubator-mxnet,jennyzhang0215/incubator-mxnet,hotpxl/mxnet,jermainewang/mxnet,danithaca/mxnet,larroy/mxnet,jiajiechen/mxnet,hpi-xnor/BMXNet,saurabh3949/mxnet,eric-haibin-lin/mxnet,rahul003/mxnet,danithaca/mxnet,reminisce/mxnet,navrasio/mxnet,xcgoner/dist-mxnet,tlby/mxnet,Ldpe2G/mxnet,Prasad9/incubator-mxnet,danithaca/mxnet,navrasio/mxnet,DR08/mxnet,navrasio/mxnet,reminisce/mxnet,luoyetx/mxnet,kevinthesun/mxnet,navrasio/mxnet,lxn2/mxnet,thirdwing/mxnet,xcgoner/dist-mxnet,jiajiechen/mxnet,ShownX/incubator-mxnet,jennyzhang0215/incubator-mxnet,Northrend/mxnet,hotpxl/mxnet,ZihengJiang/mxnet,DR08/mxnet,hotpxl/mxnet,gautamkmr/incubator-mxnet,szha/mxnet,Prasad9/incubator-mxnet,formath/mxnet,DickJC123/mxnet,reminisce/mxnet,hesseltuinhof/mxnet,sxjscience/mxnet,piiswrong/mxnet,TuSimple/mxnet,madjam/mxnet,sergeykolychev/mxnet,precedenceguo/mxnet,rahul003/mxnet,sxjscience/mxnet,xcgoner/dist-mxnet,madjam/mxnet,ShownX/incubator-mxnet,szha/mxnet,navrasio/mxnet,tornadomeet/mxnet,stefanhenneking/mxnet,wangyum/mxnet,coder-james/mxnet,rahul003/mxnet,thirdwing/mxnet,arikpoz/mxnet,arank/mxnet,LinkHS/incubator-mxnet,jamesliu/mxnet,yuruofeifei/mxnet,wangyum/mxnet,nicklhy/mxnet,szha/mxnet,tornadomeet/mxnet,apaleyes/mxnet,ptrendx/mxnet,sergeykolychev/mxnet,yuruofeifei/mxnet,vikingMei/mxnet,Prasad9/incubator-mxnet,yajiedesign/mxnet,eric-haibin-lin/mxnet,jamesliu/mxnet,sergeykolychev/mxnet,EvanzzzZ/mxnet,arank/mxnet,sxjscience/mxnet,indhub/mxnet,precedenceguo/mxnet,LinkHS/incubator-mxnet,navrasio/mxnet,zhreshold/mxnet,tlby/mxnet,arikpoz/mxnet,likelyzhao/mxnet,apaleyes/mxnet,jamesliu/mxnet,mbaijal/incubator-mxnet,likelyzhao/mxnet,nicklhy/mxnet,likelyzhao/mxnet,ptrendx/mxnet,precedenceguo/mxnet,apaleyes/mxnet,yuruofeifei/mxnet,Northrend/mxnet,solin319/incubator-mxnet,smolix/incubator-mxnet,madjam/mxnet,LinkHS/incubator-mxnet,hesseltuinhof/mxnet,piiswrong/mxnet,hesseltuinhof/mxnet,dmlc/mxnet,yuruofeifei/mxnet,ptrendx/mxnet,ShownX/incubator-mxnet,antoan2/incubator-mxnet,DR08/mxnet,luoyetx/mxnet,weleen/mxnet,reminisce/mxnet,antoan2/incubator-mxnet,jennyzhang0215/incubator-mxnet,kevinthesun/mxnet,ZihengJiang/mxnet,kkk669/mxnet,crazy-cat/incubator-mxnet,szha/mxnet,ykim362/mxnet,EvanzzzZ/mxnet,tornadomeet/mxnet,ykim362/mxnet,tlby/mxnet,fullfanta/mxnet,apache/incubator-mxnet,luoyetx/mxnet,tlby/mxnet,kevinthesun/mxnet,dmlc/mxnet,jennyzhang0215/incubator-mxnet,sergeykolychev/mxnet,arikpoz/mxnet,TuSimple/mxnet,tornadomeet/mxnet,yuruofeifei/mxnet,ykim362/mxnet,ykim362/mxnet,ZihengJiang/mxnet,apaleyes/mxnet,hesseltuinhof/mxnet,jiajiechen/mxnet,likelyzhao/mxnet,formath/mxnet,kevinthesun/mxnet,jermainewang/mxnet,ptrendx/mxnet,sxjscience/mxnet,yajiedesign/mxnet,yuruofeifei/mxnet,kevinthesun/mxnet,piiswrong/mxnet,LinkHS/incubator-mxnet,jamesliu/mxnet,tlby/mxnet,kevinthesun/mxnet,mbaijal/incubator-mxnet,CodingCat/mxnet,arank/mxnet,reminisce/mxnet,hpi-xnor/BMXNet,lxn2/mxnet,indhub/mxnet,reminisce/mxnet,ForkedReposBak/mxnet,rahul003/mxnet,hesseltuinhof/mxnet,wangyum/mxnet,saurabh3949/mxnet,arikpoz/mxnet,weleen/mxnet,saurabh3949/mxnet,jennyzhang0215/incubator-mxnet,reminisce/mxnet,DR08/mxnet,kevinthesun/mxnet,zhreshold/mxnet,piiswrong/mxnet,stefanhenneking/mxnet,formath/mxnet,EvanzzzZ/mxnet,mbaijal/incubator-mxnet,lxn2/mxnet,jiajiechen/mxnet,sxjscience/mxnet,ForkedReposBak/mxnet,saurabh3949/mxnet,jennyzhang0215/incubator-mxnet,tlby/mxnet,ShownX/incubator-mxnet,dmlc/mxnet,lxn2/mxnet,madjam/mxnet,ykim362/mxnet,hpi-xnor/BMXNet,gautamkmr/incubator-mxnet,stefanhenneking/mxnet,danithaca/mxnet,larroy/mxnet,piiswrong/mxnet,Ldpe2G/mxnet,smolix/incubator-mxnet,precedenceguo/mxnet,stefanhenneking/mxnet,Ldpe2G/mxnet,kkk669/mxnet,DR08/mxnet,ZihengJiang/mxnet,jermainewang/mxnet,luoyetx/mxnet,szha/mxnet,formath/mxnet,coder-james/mxnet,gautamkmr/incubator-mxnet,TuSimple/mxnet,wangyum/mxnet,crazy-cat/incubator-mxnet,jamesliu/mxnet,yuruofeifei/mxnet,solin319/incubator-mxnet,nicklhy/mxnet,EvanzzzZ/mxnet,mbaijal/incubator-mxnet,leezu/mxnet,szha/mxnet,larroy/mxnet,smolix/incubator-mxnet,indhub/mxnet,thirdwing/mxnet,likelyzhao/mxnet,TuSimple/mxnet,antoan2/incubator-mxnet,ZihengJiang/mxnet,dmlc/mxnet,madjam/mxnet,lxn2/mxnet,Northrend/mxnet,hotpxl/mxnet,crazy-cat/incubator-mxnet,Northrend/mxnet,nicklhy/mxnet,solin319/incubator-mxnet,weleen/mxnet,luoyetx/mxnet,DickJC123/mxnet,TuSimple/mxnet,jiajiechen/mxnet,tlby/mxnet,madjam/mxnet,EvanzzzZ/mxnet,indhub/mxnet,weleen/mxnet,TuSimple/mxnet,apaleyes/mxnet,mbaijal/incubator-mxnet,LinkHS/incubator-mxnet,Mega-DatA-Lab/mxnet,fullfanta/mxnet,reminisce/mxnet,indhub/mxnet,ptrendx/mxnet,szha/mxnet,yuruofeifei/mxnet,jamesliu/mxnet,coder-james/mxnet,leezu/mxnet,luoyetx/mxnet,larroy/mxnet,vikingMei/mxnet,EvanzzzZ/mxnet,formath/mxnet,hpi-xnor/BMXNet,jermainewang/mxnet,Prasad9/incubator-mxnet,rahul003/mxnet,danithaca/mxnet,fullfanta/mxnet,danithaca/mxnet,TuSimple/mxnet,CodingCat/mxnet,navrasio/mxnet,larroy/mxnet,apache/incubator-mxnet,Mega-DatA-Lab/mxnet,arank/mxnet,thirdwing/mxnet,zhreshold/mxnet,yajiedesign/mxnet,yuruofeifei/mxnet,saurabh3949/mxnet,dmlc/mxnet,apaleyes/mxnet,precedenceguo/mxnet,vikingMei/mxnet,antoan2/incubator-mxnet,solin319/incubator-mxnet,gautamkmr/incubator-mxnet,rahul003/mxnet,hpi-xnor/BMXNet,EvanzzzZ/mxnet,hesseltuinhof/mxnet,Prasad9/incubator-mxnet,piiswrong/mxnet,stefanhenneking/mxnet,pluskid/mxnet,wangyum/mxnet,pluskid/mxnet,formath/mxnet,ForkedReposBak/mxnet,DickJC123/mxnet,CodingCat/mxnet,yajiedesign/mxnet,madjam/mxnet,Prasad9/incubator-mxnet,coder-james/mxnet,ForkedReposBak/mxnet,CodingCat/mxnet,weleen/mxnet,ykim362/mxnet,jiajiechen/mxnet,fullfanta/mxnet,tlby/mxnet,thirdwing/mxnet,TuSimple/mxnet,zhreshold/mxnet,arikpoz/mxnet,LinkHS/incubator-mxnet,hesseltuinhof/mxnet,ShownX/incubator-mxnet,vikingMei/mxnet,coder-james/mxnet,mbaijal/incubator-mxnet,pluskid/mxnet,apaleyes/mxnet,nicklhy/mxnet,crazy-cat/incubator-mxnet,tlby/mxnet,madjam/mxnet,sergeykolychev/mxnet,jermainewang/mxnet,kkk669/mxnet,rahul003/mxnet,ykim362/mxnet,xcgoner/dist-mxnet,thirdwing/mxnet,larroy/mxnet,indhub/mxnet,lxn2/mxnet,rahul003/mxnet,jamesliu/mxnet,kkk669/mxnet,sxjscience/mxnet,tornadomeet/mxnet,reminisce/mxnet,wangyum/mxnet,Mega-DatA-Lab/mxnet,Mega-DatA-Lab/mxnet,antoan2/incubator-mxnet,Northrend/mxnet,arikpoz/mxnet,indhub/mxnet,dmlc/mxnet,ShownX/incubator-mxnet,jennyzhang0215/incubator-mxnet,sxjscience/mxnet,hotpxl/mxnet,zhreshold/mxnet,gautamkmr/incubator-mxnet,coder-james/mxnet,yajiedesign/mxnet,thirdwing/mxnet,thirdwing/mxnet,vikingMei/mxnet,Ldpe2G/mxnet,antoan2/incubator-mxnet,xcgoner/dist-mxnet,nicklhy/mxnet,stefanhenneking/mxnet,dmlc/mxnet,szha/mxnet,larroy/mxnet,saurabh3949/mxnet,kkk669/mxnet,danithaca/mxnet,arank/mxnet,ptrendx/mxnet,eric-haibin-lin/mxnet,larroy/mxnet,smolix/incubator-mxnet,fullfanta/mxnet,Northrend/mxnet,piiswrong/mxnet,leezu/mxnet,DR08/mxnet,arank/mxnet,luoyetx/mxnet,ForkedReposBak/mxnet,ykim362/mxnet,hotpxl/mxnet,indhub/mxnet,ZihengJiang/mxnet,danithaca/mxnet,hotpxl/mxnet,ZihengJiang/mxnet,Northrend/mxnet,CodingCat/mxnet,dmlc/mxnet,leezu/mxnet,pluskid/mxnet,sergeykolychev/mxnet,kkk669/mxnet,solin319/incubator-mxnet,eric-haibin-lin/mxnet,lxn2/mxnet,Mega-DatA-Lab/mxnet,leezu/mxnet,pluskid/mxnet,Ldpe2G/mxnet,smolix/incubator-mxnet,likelyzhao/mxnet,formath/mxnet,jiajiechen/mxnet,CodingCat/mxnet,ZihengJiang/mxnet,xcgoner/dist-mxnet,CodingCat/mxnet,DR08/mxnet,precedenceguo/mxnet,dmlc/mxnet,fullfanta/mxnet,Mega-DatA-Lab/mxnet,Prasad9/incubator-mxnet,arikpoz/mxnet,kkk669/mxnet,crazy-cat/incubator-mxnet,antoan2/incubator-mxnet,crazy-cat/incubator-mxnet,coder-james/mxnet,saurabh3949/mxnet,formath/mxnet,solin319/incubator-mxnet,mbaijal/incubator-mxnet,tornadomeet/mxnet,smolix/incubator-mxnet,eric-haibin-lin/mxnet,kevinthesun/mxnet,fullfanta/mxnet,ZihengJiang/mxnet,likelyzhao/mxnet,hpi-xnor/BMXNet,luoyetx/mxnet,pluskid/mxnet,indhub/mxnet,piiswrong/mxnet,leezu/mxnet,weleen/mxnet,hotpxl/mxnet,kevinthesun/mxnet,xcgoner/dist-mxnet,navrasio/mxnet,xcgoner/dist-mxnet,saurabh3949/mxnet,kkk669/mxnet,yajiedesign/mxnet,weleen/mxnet,TuSimple/mxnet,ForkedReposBak/mxnet,DickJC123/mxnet,ShownX/incubator-mxnet,zhreshold/mxnet,jermainewang/mxnet,danithaca/mxnet,eric-haibin-lin/mxnet,leezu/mxnet,antoan2/incubator-mxnet,hesseltuinhof/mxnet,solin319/incubator-mxnet,formath/mxnet,precedenceguo/mxnet,Ldpe2G/mxnet,ptrendx/mxnet,smolix/incubator-mxnet,Ldpe2G/mxnet,weleen/mxnet,pluskid/mxnet,jamesliu/mxnet,ForkedReposBak/mxnet,mbaijal/incubator-mxnet,Northrend/mxnet,crazy-cat/incubator-mxnet,vikingMei/mxnet,likelyzhao/mxnet,Mega-DatA-Lab/mxnet,hpi-xnor/BMXNet,arikpoz/mxnet,solin319/incubator-mxnet,lxn2/mxnet,yajiedesign/mxnet,ptrendx/mxnet,ForkedReposBak/mxnet,gautamkmr/incubator-mxnet,madjam/mxnet,coder-james/mxnet,CodingCat/mxnet,nicklhy/mxnet,jiajiechen/mxnet,eric-haibin-lin/mxnet,fullfanta/mxnet,stefanhenneking/mxnet,TuSimple/mxnet,Mega-DatA-Lab/mxnet,wangyum/mxnet,jamesliu/mxnet,Mega-DatA-Lab/mxnet,jermainewang/mxnet,wangyum/mxnet,yajiedesign/mxnet,coder-james/mxnet,wangyum/mxnet,szha/mxnet,zhreshold/mxnet,precedenceguo/mxnet,Prasad9/incubator-mxnet,stefanhenneking/mxnet,leezu/mxnet,eric-haibin-lin/mxnet,gautamkmr/incubator-mxnet,jennyzhang0215/incubator-mxnet