docs/conf: point to an existing settings module

docs/conf.py

 import os
 import django
 
-os.environ.setdefault("DJANGO_SETTINGS_MODULE", "swh.web.settings")
+os.environ.setdefault("DJANGO_SETTINGS_MODULE", "swh.web.settings.development")
 django.setup()
 
 from swh.docs.sphinx.conf import *  # NoQA