doc: update contribution guide to make it friendlier for x/ repos
The current contributor documentation is tailored towards contributors
to golang/go, but we have a number of increasingly popular x/ repos.
In this CL, I tried to generalize the language to make it apply to any
repository.
Also, I fixed an old link I noticed in editors.html.
Change-Id: Id9d8e448262ed8c3a67f49be5d554ca29df9d3c1
Reviewed-on: https://go-review.googlesource.com/c/go/+/234899
Run-TryBot: Rebecca Stambler <rstambler@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org> Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>