docs: Generate reference manual with gi-docgen

Technology preview of how the thing would look like.

I just partially converted one of the reference manual chapters from DocBook to Markdown (and I already felt I saw enough XML for the day). Generating the documentation this way is much easier. I don't love everything of the default theme, for example the class inheritance diagram, but it is an improvement over the DocBook stuff, and I think it will be polished over time.

Generating the ontology documentation as Markdown (with embedded HTML) should not be hard and can be easily included.

This requires adding the jinja2, markdown, markupsafe, toml, typogrify Python modules to the CI Docker images.