docs: add dbus-display documentation

Wire up the dbus-display documentation. The interface and feature is
implemented next.

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

docs/interop/dbus-display.rst

+D-Bus display
+=============
+
+QEMU can export the VM display through D-Bus (when started with ``-display
+dbus``), to allow out-of-process UIs, remote protocol servers or other
+interactive display usages.
+
+Various specialized D-Bus interfaces are available on different object paths
+under ``/org/qemu/Display1/``, depending on the VM configuration.
+
+QEMU also implements the standard interfaces, such as
+`org.freedesktop.DBus.Introspectable
+<https://dbus.freedesktop.org/doc/dbus-specification.html#standard-interfaces>`_.
+
+.. contents::
+   :local:
+   :depth: 1
+
+.. only:: sphinx4
+
+   .. dbus-doc:: ui/dbus-display1.xml
+
+.. only:: not sphinx4
+
+   .. warning::
+      Sphinx 4 is required to build D-Bus documentation.
+
+      This is the content of ``ui/dbus-display1.xml``:
+
+   .. literalinclude:: ../../ui/dbus-display1.xml
+      :language: xml

docs/interop/dbus.rst

@@ -108,3 +108,5 @@ QEMU Interfaces
 ===============
 
 :doc:`dbus-vmstate`
+
+:doc:`dbus-display`

docs/interop/index.rst

@@ -12,6 +12,7 @@ are useful for making QEMU interoperate with other software.
    bitmaps
    dbus
    dbus-vmstate
+   dbus-display
    live-block-operations
    pr-helper
    qemu-ga