Fix docs build by mocking ska_logging

There was no content for the all the devices as the module
`ska_logging` isn't installed.  It needs to be mocked.

docs/source/conf.py

@@ -27,7 +27,7 @@
 # sys.modules.update((mod_name, Mock()) for mod_name in MOCK_MODULES)
 
 autodoc_mock_imports = ['PyTango', 'tango', 'tango.server', 'run', 'command',
-                        'future', 'future.utils', 'ska_logging.configuration']
+                        'future', 'future.utils', 'ska_logging', 'ska_logging.configuration']
 
 import sys
 import os