[Docs] Wrap generate_missing_docs logic in auto_mocks block.

It should be possible to synchronize the documented modules even on
installations that don't have all the custom dependencies installed.

generate_missing_docs.py

@@ -11,6 +11,7 @@ from platypush.message.event import Event
 from platypush.message.response import Response
 from platypush.plugins import Plugin
 from platypush.utils.manifest import Manifests
+from platypush.utils.mock import auto_mocks
 
 
 def get_all_plugins():
@@ -159,6 +160,7 @@ def generate_responses_doc():
 
 
 def main():
+    with auto_mocks():
         generate_plugins_doc()
         generate_backends_doc()
         generate_events_doc()