docs: Move the gitlab-ci docs to RST.

I tried not to edit too much meaning in the process, but I did shuffle
some stuff around to work as structured documentation.

Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5510>
6 files changed