doc: first version of new contribute guide

I've reorganized the guide and rewritten large sections.

The structure is now more clear and logical, and can
be understood and navigated using the summary displayed at
the top of the page (before, the summary was confusing because
the guide contained H1s that were being ignored by the summary).

Both the initial onboarding process and the Gerrit
change submission process have been reworked to
include a concise checklist of steps that can be
read and understood in a few seconds, for people
that don't want or need to bother with details.
More in-depth descriptions have been moved into
separate sections, one per each checklist step.
This is by far the biggest improvement, as the previous
approach of having to read several pages just to understand
the requires steps was very scaring for beginners, in
addition of being harder to navigate.

GitHub pull requests have been integrated as a different
way to submit a change, suggested for first time contributors.

The review process has been described in more details,
documenting the workflow and the used conventions.

Most miscellanea have been moved into an "advanced
topics" chapter.

Paragraphs describing how to use git have been removed
to simplify reading. This guide should focus on Go contribution,
and not help users getting familiar with git, for which many
guides are available.

Change-Id: I6f4b76583c9878b230ba1d0225745a1708fad2e8
Reviewed-on: https://go-review.googlesource.com/93495Reviewed-by: Rob Pike <r@golang.org>