docs: Port parts of Clutter to gi-docgen

Depends on !2427 (merged)

Still a lot of items to port to the gi-docgen annotations, but at least this fixes a good amount of them.

Before:

https://bilelmoussaoui.pages.gitlab.gnome.org/mutter/clutter/
https://bilelmoussaoui.pages.gitlab.gnome.org/mutter/cally/

After:

https://gnome.pages.gitlab.gnome.org/-/mutter/-/jobs/2174049/artifacts/references/clutter/index.html

https://gnome.pages.gitlab.gnome.org/-/mutter/-/jobs/2174049/artifacts/references/cally/index.html