Update merge process documentation

We've built new tools to make it easier to understand what
merges are acceptable when; update this page to point to those
tools, and instead focus this more around processes and best
practices. Additionally, add instructions for how to process
merges.

Change-Id: I8f4f30eb07889631b7158e52a1844e895c6a36dc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3044525
Reviewed-by: Adrian Taylor <adetaylor@chromium.org>
Reviewed-by: Ben Mason <benmason@chromium.org>
Commit-Queue: Alex Mineer <amineer@chromium.org>
Cr-Commit-Position: refs/heads/main@{#915728}