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/gobject/meson.build

@@ -68,3 +68,27 @@ if get_option('man')
       install_dir: man1_dir)
   endforeach
 endif
+
+# gi-docgen version
+expand_content_files = [
+]
+
+gobject_gir = meson.current_source_dir() / 'GObject-2.0.gir'
+gobject_toml = configure_file(input: 'gobject.toml.in', output: 'gobject.toml', configuration: toml_conf)
+
+custom_target('gobject-docs',
+  input: [ gobject_toml, gobject_gir ],
+  output: 'gobject',
+  command: [
+    gidocgen,
+    'generate',
+    gidocgen_common_args,
+    '--config=@INPUT0@',
+    '--output-dir=@OUTPUT@',
+    '--content-dir=@0@'.format(meson.current_source_dir()),
+    '--add-include-path=@0@'.format(meson.current_source_dir() / '../glib'),
+    '@INPUT1@',
+  ],
+  build_by_default: true,
+  depend_files: expand_content_files,
+)