Migrate to gi-docgen

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.

Testing Done:
Ran locally and via the new convey plan.

Reviewed at https://reviews.imfreedom.org/r/905/