Update Doxygen reference to canonical URL for auto-generated documentation

Currently our Doxygen output says (For an up-to-date rendered copy of this documentation, see https://src-ref.docs.torproject.org/tor/index.html .).

We need to update this link, but let's see what the solution of tpo/tpa/team#40270 (closed) becomes first. We may want to link just to the index OR directly to the Doxygen output from the Gitlab pages hosted files.