Add links to github file to all docs (not just API docs)

[coffeemug]

And make them pretty icons (instead of a bar like in the API docs)!

Also, API docs should show links to edit doc entries that don't have their own page.

[rchordiya]

I think this issue is already resolved by providing GitHub link at the bottom of every docs under the
heading "Star this project on GitHub" am I right? or do we need to add something else?

[mglukhovsky]

@rchordiya: this issue is about linking directly to the source on GitHub of the documentation article being viewed. This would require some work on the Jekyll includes used on rethinkdb/docs.