Revamp "third-party code" docs
Revamp "third-party code" docs

Summary:
Refreshes this docs page to:

* Use consistent "library definition"/"libdef" terminology
* Refer to the `flow-typed` for public library library definitions
* Recommend using the built-in `<PROJECT_ROOT>/flow-typed/` directory (rather than making your own + writing config)
* Fix the damn CSS on the site to make inline `<code>` text stand apart better from the rest of the text.
Closes https://github.com/facebook/flow/pull/2514

Reviewed By: samwgoldman

Differential Revision: D3909756

Pulled By: jeffmo

fbshipit-source-id: 1396118f3c951b9141ccd3880eaf28db4447577a

AgentME/flow,mroch/flow,claudiopro/flow,fletcherw/flow,facebook/flow,JonathanUsername/flow,MichaelDeBoey/flow,ylu1317/flow,gabro/flow,mroch/flow,gabelevi/flow,gabro/flow,jgrund/flow,nmote/flow,AprilArcus/flow,jamesgpearce/flow,gabelevi/flow,AprilArcus/flow,AprilArcus/flow,AgentME/flow,nmote/flow,ForbesLindesay/flow,AprilArcus/flow,fletcherw/flow,JonathanUsername/flow,MichaelDeBoey/flow,popham/flow,JonathanUsername/flow,JonathanUsername/flow,jgrund/flow,ForbesLindesay/flow,MichaelDeBoey/flow,ylu1317/flow,claudiopro/flow,ylu1317/flow,AprilArcus/flow,JonathanUsername/flow,mroch/flow,JonathanUsername/flow,jgrund/flow,samwgoldman/flow,samwgoldman/flow,ForbesLindesay/flow,nmote/flow,claudiopro/flow,gabelevi/flow,claudiopro/flow,facebook/flow,facebook/flow,nmote/flow,AgentME/flow,MichaelDeBoey/flow,popham/flow,JonathanUsername/flow,TiddoLangerak/flow,facebook/flow,jgrund/flow,TiddoLangerak/flow,samwgoldman/flow,popham/flow,ylu1317/flow,gabro/flow,claudiopro/flow,samwgoldman/flow,jamesgpearce/flow,gabelevi/flow,gabelevi/flow,ForbesLindesay/flow,popham/flow,mroch/flow,facebook/flow,fletcherw/flow,TiddoLangerak/flow,ForbesLindesay/flow,jamesgpearce/flow,mroch/flow,gabelevi/flow,gabelevi/flow,samwgoldman/flow,fletcherw/flow,nmote/flow,mroch/flow,TiddoLangerak/flow,facebook/flow,AprilArcus/flow,nmote/flow,ylu1317/flow,samwgoldman/flow,jamesgpearce/flow,jgrund/flow,nmote/flow,popham/flow,AgentME/flow,gabro/flow,gabro/flow,AgentME/flow,MichaelDeBoey/flow,jgrund/flow,jamesgpearce/flow,samwgoldman/flow,fletcherw/flow,jamesgpearce/flow,TiddoLangerak/flow,fletcherw/flow,ForbesLindesay/flow,popham/flow,facebook/flow,popham/flow,gabro/flow,mroch/flow,jamesgpearce/flow