Handle GI-Docgen comments
Many GNOME components have switched to GI-Docgen which use a different format to render the documentation, it would be nice for libvaladoc to be able to parse it (so that https://valadoc.org/gtk4/Gtk.Accessible.html would actually render links) and also generate it in the .gir so that we can also use gi-docgen with Vala .girs