gitlab-ci: Use rules: instead of except:/only: for test-docs job

gitlab-ci: Use rules: instead of except:/only: for test-docs job

Only run the job automatically for Marge Bot, otherwise let it be
triggered manually.

v2:
* Never run this job for the main project, since it's only needed in
  pre-merge pipelines.
* Add comment explaining that cases not covered by explicit rules
  default to "when: never".

Reviewed-by: Daniel Stone <daniels@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5469>