GerritAccess: overhaul approvers section

I've rearranged and expanded the details of submit requirements, code
review, and submission to paint a more complete picture of the code
review process.

The primary purpose here is to clarify encouragement to vote
Auto-Submit+1. We want to encourage folks to vote Auto-Submit+1 to
ensure CLs get submitted promptly.

Change-Id: I6a6a636cce3616177cf0dd200aac6a2d87d1062a
Reviewed-on: https://go-review.googlesource.com/c/wiki/+/742820
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Auto-Submit: Michael Pratt <mpratt@google.com>
Commit-Queue: Michael Pratt <mpratt@google.com>