doc: mock vxi11 module

On Python 3.13, the vxi11 module import fails:

  autodoc: failed to import module 'siglent' from module 'labgrid.driver.power'; the following exception was raised:
  No module named 'xdrlib'

It uses xdrlib which was dropped from Python's standard library with
Python 3.13. See labgrid-project#1507
for more details.

For the time being, mock the vxi11 module to make the doc builds work on
Python 3.13.

Signed-off-by: Bastian Krause <bst@pengutronix.de>

doc/conf.py

@@ -183,7 +183,8 @@
 }
 autodoc_mock_imports = ['onewire',
                         'gi',
-                        'gi.repository',]
+                        'gi.repository',
+                        'vxi11']
 
 # -- Options for autosection ----------------------------------------------
 autosectionlabel_prefix_document = True