docs: Add initial support for using gi-docgen for docs

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

docs/reference/gio/gio.toml.in

@@ -0,0 +1,47 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
+# Copyright 2023 Matthias Clasen
+# Copyright 2023 Philip Withnall
+
+[library]
+name = "Gio"
+version = "@VERSION@"
+browse_url = "https://gitlab.gnome.org/GNOME/glib/"
+repository_url = "https://gitlab.gnome.org/GNOME/glib.git"
+website_url = "https://www.gtk.org"
+docs_urls = "https://docs.gtk.org/gio/"
+authors = "GLib Development Team"
+license = "LGPL-2.1-or-later"
+description = "Gio is a library providing useful classes for general purpose I/O, networking, IPC, settings, and other high level application functionality"
+dependencies = [ "GLib-2.0", "GModule-2.0", "GObject-2.0" ]
+devhelp = true
+search_index = true
+
+  [dependencies."GLib-2.0"]
+  name = "GLib"
+  description = "The base utility library"
+  docs_url = "https://docs.gtk.org/glib/"
+
+  [dependencies."GModule-2.0"]
+  name = "GModule"
+  description = "Portable API for dynamically loading modules"
+  docs_url = "https://docs.gtk.org/gmodule/"
+
+  [dependencies."GObject-2.0"]
+  name = "GObject"
+  description = "The base type system library"
+  docs_url = "https://docs.gtk.org/gobject/"
+
+[theme]
+name = "basic"
+show_index_summary = true
+show_class_hierarchy = true
+
+[extra]
+urlmap_file = "urlmap.js"
+# The same order will be used when generating the index
+content_files = [
+]
+content_images = [
+  "menu-example.png",
+  "menu-model.png",
+]