doc: Use GitLab MRs for patches, not the list

Though Wayland and the protocols still use mail-based patch review,
Weston can now move to GitLab MRs with review through that system.

Add some documentation on how to submit patches through GitLab,
specifically targeted at people who may be familiar with GitLab review,
but not familiar with our rebasing microcommit workflow.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Quentin Glidic <sardemff7+git@sardemff7.net>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>