Make sure docs only merge requests can run CI

Background

!24568 (merged) broke docs only merge requests pipeline because it added needs: [retrieve-tests-metadata] to various jobs. The attempt was to run them earlier so they're not blocked by the newly added Fixtures stage.

However, docs only merge requests do not have retrieve-tests-metadata job, and this can potentially apply to other jobs if they don't have retrieve-tests-metadata either.

What does this MR do?

Actually, we can just pass needs: [] because they really don't need anything.

Try to replicate the same pipeline as in https://gitlab.com/gitlab-org/gitlab/pipelines/119123894

Testing docs only merge request targeting this branch: !25614 (closed)

This also addresses #207374 (closed) while I am at it.

Screenshots

Showing that we can run those tests without anything from setting up:

Closes #207388 (closed)