Improve quickstart docs
Improve quickstart docs

See https://github.com/jekyll/jekyll/pull/5630 for more details on the update. 

@jekyll/documentation
@DirtyF
alex-kovac/jekyll,chrisfinazzo/jekyll,jaybe-jekyll/jekyll,tomjohnson1492/jekyll,ZDroid/jekyll,chrisfinazzo/jekyll,fulldecent/jekyll,ZDroid/jekyll,alex-kovac/jekyll,alex-kovac/jekyll,jaybe-jekyll/jekyll,darwin/jekyll,rasa2011/rasa2011.github.io,darwin/jekyll,chrisfinazzo/jekyll,x-way/jekyll,tillgrallert/jekyll,fulldecent/jekyll,Strangehill/jekyll,ryanshaw/jekyll,fulldecent/jekyll,chrisfinazzo/jekyll,rlugojr/jekyll,jekyll/jekyll,Strangehill/jekyll,gencer/jekyll,alex-kovac/jekyll,Strangehill/jekyll,xiebinhqy/jklly_china,xiebinhqy/jklly_china,ryanshaw/jekyll,tomjohnson1492/jekyll,rasa2011/rasa2011.github.io,alex-kovac/jekyll,gencer/jekyll,getandpost/jekyll.github.io,getandpost/jekyll.github.io,x-way/jekyll,tillgrallert/jekyll,gencer/jekyll,jaybe-jekyll/jekyll,Strangehill/jekyll,gencer/jekyll,x-way/jekyll,rasa2011/rasa2011.github.io,darwin/jekyll,tomjohnson1492/jekyll,yhironaka/yhironaka.github.io,rlugojr/jekyll,ryanshaw/jekyll,ryanshaw/jekyll,ZDroid/jekyll,jekyll/jekyll,tillgrallert/jekyll,jaybe-jekyll/jekyll,rlugojr/jekyll,Strangehill/jekyll,tomjohnson1492/jekyll,tillgrallert/jekyll,xiebinhqy/jklly_china,tillgrallert/jekyll,gencer/jekyll,tomjohnson1492/jekyll,jekyll/jekyll,xiebinhqy/jklly_china,jekyll/jekyll,fulldecent/jekyll,darwin/jekyll,x-way/jekyll,rlugojr/jekyll,getandpost/jekyll.github.io