[release-branch.go1.10] doc: sync with master

A number of changes were applied to documentation in master, including
a wide rewrite of the contribution guide. Backport them all to get them
deployed on golang.org.

$ git ch master -- doc
$ git show-ref master
88756931 refs/heads/master

Change-Id: Ib45ff191f3b60723aa4421113992289c37d144aa
Reviewed-on: https://go-review.googlesource.com/114503


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Andrew Bonventre <andybons@golang.org>
Run-TryBot: Andrew Bonventre <andybons@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>