Also called "feature" option in meson, so that by default it depends on
auto-detection of the gi-docgen hence won't break the configuration when
the tool is missing (the feature is simply disabled).
Also move the program check into the root meson file, which is anyway
much better to have a better visibility of features, otherwise we'd end
up just having tests everywhere in any possible random directory of the
repo.
Finally add a line in the summary of the configuration step, displaying
the docs generation being enabled or disabled.
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
