Deploy docs directly & fix linkcheck

What does this MR do?

This MR adjusts the CI and the corresponding GitLab Pages settings (and the DNS settings for our domain) to directly deploy the documentation to docs.utopia-project.org

Furthermore, two minor tweaks are introduced:

• sphinx linkcheck now ignores those URLs that are perpetually broken (e.g. the relative doxygen links, which linkcheck doesn't like).
  • This now allows to set the sphinx_linkcheck target to "fail on warn", such that we get notified upon broken links 🎉
• Minor tweak to upgrade instructions for Utopia

Is there something that needs to be double checked?

• Ok to have linkcheck fail on errors? Is that robust enough?

Can this MR be accepted?

• Implemented the changes
• Tests
  • Tested manually: Docs deployment works
  • Tested manually: linkcheck works
  • Via CI: linkcheck
• Checked test code coverage on new and adjusted code: no change
• Added or updated documentation (all internal)
• Reasonably up-to-date with current master
• Pipeline passing without warnings
• History cleaned-up or squash option set
• Reviewed and approved by @tgaskin

Related issues

Closes #351 (closed)