ci: actually check formatting

1. The page preview seems unusable anyway, as GitLab doesn't serve HTML
   pages (only allows to download them).
2. Rules seems broken and 'splits' pipelines:
   https://gitlab.com/gitlab-org/gitlab/-/issues/34756
   In return, 'only' is deprecated, but at least it works.

This also hopefully stops us from submitting random MR docs onto the
production pages.