docs: Move the current CI .rst doc to docs/ci/ and link to it from .gitlab-ci.

docs: Move the current CI .rst doc to docs/ci/ and link to it from .gitlab-ci.

I want the docs to be discoverable next to the code, and sphinx insists
that all docs are under the top-level docs dir (sigh).  We can't symlink
from that dir to .gitlab-ci because windows builds can't do symlinks, so
link back the other direction.

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>