gi-docgen is apparently the future and it works much faster with no extra work like updating gplugin-docs.xml that gtk-doc required.
However, there are a few additional python dependencies for gi-docgen, notably python3-jinja2, python3-pygments, python3-toml, python3-typogrify. I'm in the process of updating the builders right now as the docs build by default now.

Ran locally and via the new convey plan.