build: Add more apps to the set of installed apps.

These app models get pulled in when trying to build the OpenAPI docs so
we need their relevant apps installed into the environment used to
generate the docs.

docs/docs_settings.py

@@ -38,6 +38,13 @@ INSTALLED_APPS.extend(
         "cms.djangoapps.xblock_config.apps.XBlockConfig",
         "lms.djangoapps.lti_provider",
         "openedx.core.djangoapps.content.search",
+        "openedx.core.djangoapps.content_libraries",
+        "openedx.core.djangoapps.content_staging",
+        "openedx.core.djangoapps.bookmarks",
+        "openedx.core.djangoapps.discussions",
+        "openedx.core.djangoapps.theming",
+        "lms.djangoapps.program_enrollments",
+        "push_notifications",
     ]
 )