gtk-doc support was removed around 3 years ago with commit 7dc0b0e6.
Now that we have Meson and [MRs related to documentation], it makes
sense to re-enable this, so people who want to use libmutter can do so
without having to actually dive in the code.
[MRs related to documentation]: https://gitlab.gnome.org/GNOME/mutter/merge_requests/269
