docs/sphinx: add sphinx modules to include D-Bus documentation

Add a new dbus-doc directive to import D-Bus interfaces documentation
from the introspection XML. The comments annotations follow the
gtkdoc/kerneldoc style, and should be formatted with reST.

Note: I realize after the fact that I was implementing those modules
with sphinx 4, and that we have much lower requirements. Instead of
lowering the features and code (removing type annotations etc), let's
have a warning in the documentation when the D-Bus modules can't be
used, and point to the source XML file in that case.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>

docs/sphinx/fakedbusdoc.py

@@ -0,0 +1,25 @@
+# D-Bus XML documentation extension, compatibility gunk for <sphinx4
+#
+# Copyright (C) 2021, Red Hat Inc.
+#
+# SPDX-License-Identifier: LGPL-2.1-or-later
+#
+# Author: Marc-André Lureau <marcandre.lureau@redhat.com>
+"""dbus-doc is a Sphinx extension that provides documentation from D-Bus XML."""
+
+from sphinx.application import Sphinx
+from sphinx.util.docutils import SphinxDirective
+from typing import Any, Dict
+
+
+class FakeDBusDocDirective(SphinxDirective):
+    has_content = True
+    required_arguments = 1
+
+    def run(self):
+        return []
+
+
+def setup(app: Sphinx) -> Dict[str, Any]:
+    """Register a fake dbus-doc directive with Sphinx"""
+    app.add_directive("dbus-doc", FakeDBusDocDirective)