Eventually we want to have all the high-level code documentation in the
component API reference documentation. However, gi-docgen is currently
missing support for mermaid so we just keep the files in `doc/` and link
to them from `code-overview.md`.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3505>
gtk-doc support was taken out about six years ago, but never replaced.
Add support for the newer gi-docgen, which should hopefully encourage
both C and GJS users of these libraries to improve the reference.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2248>