move doxygen docs to GitLab CI or deprecate

in tpo/tpa/team#40225 (closed), i was asked to "remove or disable Tor jenkins builders". while doing that, I noticed that doxgen docs are not currently build in a known location in GitLab CI:

https://src-ref.docs.torproject.org/tor/index.html

so retiring those would mean leaving that stale website in place for the foreseeable future.

could you move that to GitLab CI and GitLab pages? the following should get you started:

https://gitlab.torproject.org/tpo/tpa/team/-/wikis/howto/gitlab/#publishing-gitlab-pages

if not, do let me know what's your blocker. i believe the resulting URL for core tor would be:

https://tpo.pages.torproject.net/core/tor/

once that is done we can retire the jenkins job, and then provide a redirect to the gitlab pages.

makes sense?