Add a note about CI assets required by outside projects

The following discussion from !26008 (merged) should be addressed:

• @rymai started a discussion: (+3 comments)

  That's actually the only assets needed by the pages job!

  We should probably add a note in .gitlab/ci/frontend.gitlab-ci.yml then (let's resolve this as a follow-up issue).

We ought to add some comments to our CI configuration regarding assets that are consumed within other projects (e.g. application.css within gitlab-ui) to ensure they are not removed.

/cc @leipert