docs(contributing) various improvements
docs(contributing) various improvements

* defer "welcome to the community" gateway-type of sentences to the main
  Kong/kong contributing file
* improve phrasing in various places
* respect 80 col limit
* more git commit format scopes and context
* add missing ToC links
* remove git instructions that seem "extraneous" for a simple
  documentation website (they apply better to a complex codebase like that
  of Kong/kong where those excerpts were taken from)

From #640
Mashape/getkong.org,Mashape/getkong.org,Mashape/getkong.org