Add documentation for collection static files
Add documentation for collection static files

Any file without valid front matter in a collection is available via
`collection.files`.

Inspired by #3583.

ZainRizvi/jekyll,Strangehill/jekyll,jmptrader/jekyll,marrujoalex/jekyll,stomar/jekyll,UniFreak/jekyllcn,jaroot32/jekyll,rlugojr/jekyll,nasht00/jekyll,fulldecent/jekyll,CorainChicago/jekyll,goragod/jekyll,rlugojr/jekyll,desidude03/jekyll,howkj1/jekyll,MauroMrod/jekyll,Tiger66639/jekyll,dangaute/eng,zsyed91/jekyll,winndows/jekyll,howkj1/jekyll,krahman/jekyll,CheekyFE/jekyll,matuzo/jekyll,stomar/jekyll,jmhardison/jekyll,darwin/jekyll,xcatliu/jekyllcn,cyberid41/jekyll,eloyesp/jekyll,tareq-s/jekyll,omeripek/jekyll,drobati/jekyll,omeripek/jekyll,jmepg/jekyll,likong/jekyll,FlyingWHR/jekyll,kuangyeheng/jekyll,krahman/jekyll,ZainRizvi/jekyll,naughtyboy83/jekyll,vcgato29/jekyll,stomar/jekyll,CheekyFE/jekyll,alihalabyah/jekyll,imaustink/jekyll,xiebinhqy/jklly_china,princeofdarkness76/jekyll,xiebinhqy/jklly_china,darwin/jekyll,SQS2/Test,jmptrader/jekyll,likong/jekyll,xantage/jekyll,ReachingOut/jekyll,Teino1978-Corp/Teino1978-Corp-jekyll,wentixiaogege/jekyll,mnuessler/jekyll,ZainRizvi/jekyll,tamouse/jekyll,dezon/jekyll,LuoPX-15510486969/jekyll,zhangkuaiji/jekyllcn,teju111/jekyll,htmelvis/jekyll,chiyodad/jekyll,xtymichael/jekyll,howkj1/jekyll,xiongjungit/jekyll,chrisfinazzo/jekyll,wadaries/jekyll,rongl/jekyll,aarvay/jekyll,isathish/jekyll,royalwang/jekyll,abhilashhb/jekyll,honger05/jekyllcn,floydpraveen/jekyll,thejameskyle/jekyll,hartmel/jekyll,daniel-beck/jekyll,greent2008/jekyll,shuber2/jekyll,tomasdiez/jekyll,18F/jekyll,EddieDow/jekyll,LuoPX-15510486969/jekyll,gkunwar/gkunwar.github.io,dezon/jekyll,rovrevik/jekyll,xiaoshaozi52/jekyll,nickg33/jekyll,richsoni/jekyll,Teino1978-Corp/Teino1978-Corp-jekyll,foocoder/jekyll,rajrathore/jekyll,kuangyeheng/jekyll,MauroMrod/jekyll,mnuessler/jekyll,xiaoshaozi52/jekyll,ls2uper/jekyll,rasa2011/rasa2011.github.io,sanxore/sanxore.github.com,krishnakalyan3/jekyll,nickg33/jekyll,indraj/jekyll,tugberkugurlu/jekyll,chiyodad/jekyll,gynter/jekyll,princeofdarkness76/jekyll,wadaries/jekyll,18F/jekyll,daniel-beck/jekyll,floydpraveen/jekyll,rlugojr/jekyll,vidbina/jekyll,alihalabyah/jekyll,derekgottlieb/jekyll,ewinkler/jekyll,jaybe-jekyll/jekyll,greent2008/jekyll,CPVPLachlan/CPVP-Blog,digideskio/jekyll,Tyrion22/jekyll,zsyed91/jekyll,mrb/jekyll,jeffkole/jekyll,noikiy/jekyll,UniFreak/jekyllcn,AnanthaRajuC/jekyll,pheuko/jekyll,jmknoll/jekyll,dansef/jekyll,harrissoerja/jekyll,tomasdiez/jekyll,hubsaysnuaa/jekyll,johnnycastilho/jekyll,saitodisse/jekyll,vidbina/jekyll,ls2uper/jekyll,chrisfinazzo/jekyll,darwin/jekyll,jaybe-jekyll/jekyll,nemotan/jekyll,m2candre/jekyll,xcatliu/jekyllcn,zhangkuaiji/jekyllcn,iRoxx/jekyll,tugberkugurlu/jekyll,tylermachen/jekyll,xantage/jekyll,yihangho/jekyll,xcatliu/jekyllcn,tylermachen/jekyll,liukaijv/jekyll,thetaxman/jekyll,tillgrallert/jekyll,adilapapaya/jekyll,mnuessler/jekyll,liukaijv/jekyll,tomasdiez/jekyll,gaosboy/jekyll,gynter/jekyll,penchan1218/jekyllcn,vcgato29/jekyll,nateberkopec/jekyll,x-way/jekyll,jmknoll/jekyll,yhironaka/yhironaka.github.io,gaosboy/jekyll,thetaxman/jekyll,CPVPLachlan/CPVP-Blog,jaybe-jekyll/jekyll,pmarsceill/jekyll,xcatliu/jekyllcn,Tiger66639/jekyll,tillgrallert/jekyll,likong/jekyll,teju111/jekyll,FlyingWHR/jekyll,sasongkojati/jekyll,zsyed91/jekyll,codeclimate-testing/jekyll,ls2uper/jekyll,AaronSikes/jekyll,Kinghack/kinghack.github.com,adilapapaya/jekyll,fulldecent/jekyll,tomjohnson1492/jekyll,julienbourdeau/jekyll,fengsmith/jekyllcn,AnanthaRajuC/jekyll,hiteshsuthar/jekyll,jekyll/jekyll,omeripek/jekyll,greent2008/jekyll,htmelvis/jekyll,ramoslin02/jekyll,wadaries/jekyll,angeliaz/jekyll,rovrevik/jekyll,saitodisse/jekyll,AtekiRyu/jekyll,doubleday/jekyll,rlugojr/jekyll,PepSalehi/jekyll,eninya/jekyll,kimeng/jekyll,mrb/jekyll,tamouse/jekyll,adilapapaya/jekyll,jeffkole/jekyll,mixxmac/jekyll,EddieDow/jekyll,nasht00/jekyll,penchan1218/jekyllcn,LeuisKen/jekyllcn,Wirachmat/jekyll,SQS2/Test,tomjohnson1492/jekyll,aarvay/jekyll,gaosboy/jekyll,tobscure/jekyll,leichunxin/jekyll,digideskio/jekyll,harrissoerja/jekyll,mixxmac/jekyll,zhangkuaiji/jekyllcn,fulldecent/jekyll,darwin/jekyll,Kinghack/kinghack.github.com,aarvay/jekyll,Tyrion22/jekyll,thejameskyle/jekyll,iRoxx/jekyll,fengsmith/jekyll,jmhardison/jekyll,angeliaz/jekyll,fabulousu/jekyll,royalwang/jekyll,jaroot32/jekyll,fabulousu/jekyll,noikiy/jekyll,eloyesp/jekyll,Teino1978-Corp/jekyll,rasa2011/rasa2011.github.io,shuber2/jekyll,chinayin/jekyll,Teino1978-Corp/Teino1978-Corp-jekyll,codeclimate-testing/jekyll,backendeveloper/jekyll,cyberid41/jekyll,gencer/jekyll,naughtyboy83/jekyll,floydpraveen/jekyll,oresmus/jekyll,alex-kovac/jekyll,AtekiRyu/jekyll,garvitr/jekyll,martynbm/jekyll,goragod/jekyll,dansef/jekyll,x-way/jekyll,larryfox/jekyll,foocoder/jekyll,fengsmith/jekyll,krahman/jekyll,ajhit406/jekyll,winndows/jekyll,dansef/jekyll,shliujing/jekyll,oresmus/jekyll,eloyesp/jekyll,xiebinhqy/jklly_china,rasa2011/rasa2011.github.io,gencer/jekyll,rovrevik/jekyll,indraj/jekyll,drobati/jekyll,AaronSikes/jekyll,alihalabyah/jekyll,chinayin/jekyll,harrissoerja/jekyll,wentixiaogege/jekyll,hartmel/jekyll,julienbourdeau/jekyll,rebornix/jekyll,backendeveloper/jekyll,bsmr-ruby/jekyll,tareq-s/jekyll,xantage/jekyll,rajrathore/jekyll,jmhardison/jekyll,julienbourdeau/jekyll,nickg33/jekyll,fengsmith/jekyllcn,chrisfinazzo/jekyll,jekyll/jekyll,krishnakalyan3/jekyll,Teino1978-Corp/jekyll,rongl/jekyll,MjAbuz/jekyll,0x00evil/jekyll,shliujing/jekyll,nateberkopec/jekyll,tugberkugurlu/jekyll,thejameskyle/jekyll,richsoni/jekyll,indraj/jekyll,18F/jekyll,alex-kovac/jekyll,tareq-s/jekyll,stomar/jekyll,sasongkojati/jekyll,imaustink/jekyll,Tyrion22/jekyll,richsoni/jekyll,ewinkler/jekyll,chrisfinazzo/jekyll,kimeng/jekyll,tobscure/jekyll,ReachingOut/jekyll,ZDroid/jekyll,derekgottlieb/jekyll,alfredxing/jekyll,shuber2/jekyll,teju111/jekyll,0x00evil/jekyll,chiyodad/jekyll,xiongjungit/jekyll,nasht00/jekyll,gencer/jekyll,princeofdarkness76/jekyll,honger05/jekyllcn,ramoslin02/jekyll,tillgrallert/jekyll,tamouse/jekyll,desidude03/jekyll,xiebinhqy/jklly_china,Strangehill/jekyll,royalwang/jekyll,brint/jekyll,Teino1978-Corp/jekyll,LeuisKen/jekyllcn,SQS2/Test,cyberid41/jekyll,martynbm/jekyll,htmelvis/jekyll,drobati/jekyll,ReachingOut/jekyll,alfredxing/jekyll,bsmr-ruby/jekyll,christophermanning/jekyll,daniel-beck/jekyll,dezon/jekyll,sanxore/sanxore.github.com,QuinntyneBrown/jekyll,egobrightan/jekyll,saitodisse/jekyll,PepSalehi/jekyll,dangaute/eng,oresmus/jekyll,honger05/jekyllcn,hiteshsuthar/jekyll,gkunwar/gkunwar.github.io,naughtyboy83/jekyll,0x00evil/jekyll,fulldecent/jekyll,tomjohnson1492/jekyll,ryanshaw/jekyll,tomjohnson1492/jekyll,jeffkole/jekyll,garvitr/jekyll,bsmr-ruby/jekyll,nemotan/jekyll,LeuisKen/jekyllcn,xiaoshaozi52/jekyll,christophermanning/jekyll,ajhit406/jekyll,desidude03/jekyll,m2candre/jekyll,leichunxin/jekyll,CorainChicago/jekyll,getandpost/jekyll.github.io,hubsaysnuaa/jekyll,chinayin/jekyll,doubleday/jekyll,alfredxing/jekyll,nateberkopec/jekyll,garvitr/jekyll,xtymichael/jekyll,anthonyrosengren/grasshoppergathering2.github-io,sanxore/sanxore.github.com,rebornix/jekyll,PepSalehi/jekyll,noikiy/jekyll,nasht00/jekyll,tillgrallert/jekyll,superve/jekyll,Wirachmat/jekyll,superve/jekyll,QuinntyneBrown/jekyll,mixxmac/jekyll,pheuko/jekyll,LuoPX-15510486969/jekyll,Tiger66639/jekyll,winndows/jekyll,fengsmith/jekyll,abhilashhb/jekyll,ajhit406/jekyll,goragod/jekyll,m2candre/jekyll,Wirachmat/jekyll,egobrightan/jekyll,x-way/jekyll,backendeveloper/jekyll,fengsmith/jekyllcn,dangaute/eng,imaustink/jekyll,johnnycastilho/jekyll,abhilashhb/jekyll,matuzo/jekyll,ducktyper/jekyll,mrb/jekyll,gynter/jekyll,ramoslin02/jekyll,larryfox/jekyll,ryanshaw/jekyll,brint/jekyll,pheuko/jekyll,foocoder/jekyll,egobrightan/jekyll,alfredxing/jekyll,larryfox/jekyll,jaroot32/jekyll,doubleday/jekyll,supriyantomaftuh/jekyll,Kinghack/kinghack.github.com,Strangehill/jekyll,ducktyper/jekyll,fabulousu/jekyll,ducktyper/jekyll,matuzo/jekyll,isathish/jekyll,isathish/jekyll,getandpost/jekyll.github.io,vcgato29/jekyll,sasongkojati/jekyll,jekyll/jekyll,jmepg/jekyll,rongl/jekyll,alex-kovac/jekyll,krishnakalyan3/jekyll,johnnycastilho/jekyll,vidbina/jekyll,trungvothanh/jekyll,jaybe-jekyll/jekyll,AaronSikes/jekyll,jmptrader/jekyll,supriyantomaftuh/jekyll,CheekyFE/jekyll,derekgottlieb/jekyll,gkunwar/gkunwar.github.io,jeffkole/jekyll,MauroMrod/jekyll,CorainChicago/jekyll,gencer/jekyll,xtymichael/jekyll,nemotan/jekyll,alex-kovac/jekyll,trungvothanh/jekyll,x-way/jekyll,superve/jekyll,pmarsceill/jekyll,kimeng/jekyll,hubsaysnuaa/jekyll,yihangho/jekyll,liukaijv/jekyll,ryanshaw/jekyll,eninya/jekyll,tillgrallert/jekyll,yihangho/jekyll,FlyingWHR/jekyll,xiongjungit/jekyll,codeclimate-testing/jekyll,martynbm/jekyll,hartmel/jekyll,leichunxin/jekyll,trungvothanh/jekyll,gencer/jekyll,CPVPLachlan/CPVP-Blog,kuangyeheng/jekyll,anthonyrosengren/grasshoppergathering2.github-io,UniFreak/jekyllcn,ZDroid/jekyll,MjAbuz/jekyll,wentixiaogege/jekyll,rebornix/jekyll,alex-kovac/jekyll,MjAbuz/jekyll,EddieDow/jekyll,bsmr-ruby/jekyll,princeofdarkness76/jekyll,iRoxx/jekyll,QuinntyneBrown/jekyll,jmknoll/jekyll,anthonyrosengren/grasshoppergathering2.github-io,eninya/jekyll,penchan1218/jekyllcn,ryanshaw/jekyll,ZDroid/jekyll,marrujoalex/jekyll,tylermachen/jekyll,noikiy/jekyll,floydpraveen/jekyll,derekgottlieb/jekyll,AtekiRyu/jekyll,Strangehill/jekyll,mrb/jekyll,digideskio/jekyll,shliujing/jekyll,angeliaz/jekyll,ajhit406/jekyll,tobscure/jekyll,AnanthaRajuC/jekyll,pmarsceill/jekyll,christophermanning/jekyll,pmarsceill/jekyll,supriyantomaftuh/jekyll,hiteshsuthar/jekyll,marrujoalex/jekyll,thetaxman/jekyll,brint/jekyll,getandpost/jekyll.github.io,tomjohnson1492/jekyll,rajrathore/jekyll,jekyll/jekyll,brint/jekyll,zsyed91/jekyll,ewinkler/jekyll,jmepg/jekyll,Strangehill/jekyll