The GIR files are now built by GLib itself, so they will be in the build
directories of each sub-library, except for GLib-2.0 which is built
alongside GObject-2.0.
Gate the API reference on the availability of the introspection data as
well, so we don't accidentally try and generate the documentation
without a description of our API.
The files here are copied from the docs-gtk-org
branch of gtk.
This adds gi-docgen to the CI Dockerfiles and ensures the new versions
(including the OS upgrades from the previous commit) are used during CI.
Helps: #3037
