gtk-doc has been slowly dying for the past few years; with gi-docgen we
have a nice successor.
This also makes sure the C documentation also uses the GIR file, which
in turn means faster build times (since all the C code doesn't have to
be parsed and recompiled again), and has a clear dependency graph.
See the [gi-docgen tutorial] for more info on how the system works.
[gi-docgen tutorial]: https://gnome.pages.gitlab.gnome.org/gi-docgen/tutorial.html
