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

Guneet-Dhillon/mxnet,Guneet-Dhillon/mxnet,Guneet-Dhillon/mxnet,Guneet-Dhillon/mxnet,Guneet-Dhillon/mxnet,Guneet-Dhillon/mxnet,Guneet-Dhillon/mxnet,Guneet-Dhillon/mxnet,Guneet-Dhillon/mxnet