docs: Update stable process around using fixes: and gitlab
Currently the docs still recommend using mesa-stable@lists.freedesktop.org, which is pretty awful. We really don't want a second mailing list and it's mostly full of junk because of CC: tags anyway.
This changes the preferred actions to be:
- use a fixes: tag ahead of time
- use a Cc tag ahead of time if fixes isn't appropriate
- Use a gitlab MR against the staging/ branch for post-merge/backport nominations