sphinx: remove usage of deprecated notfound-page options

sphinx-notfound-page deprecated and removed notfound_no_urls_prefix,
notfound_default_language, and notfound_default_version in favor of
notfound_urls_prefix.
We were still using these old options which caused our 404 pages to
break.

docs/docsite/rst/conf.py

@@ -333,15 +333,10 @@
 # with no 'notfound_template' and no 'notfound_context' set,
 # the extension builds 404.rst into a location-agnostic 404 page
 #
-# default is `en` - using this for the sub-site:
-notfound_default_language = "ansible"
-# default is `latest`:
 # setting explicitly - docsite serves up /ansible/latest/404.html
 # so keep this set to `latest` even on the `devel` branch
 # then no maintenance is needed when we branch a new stable_x.x
-notfound_default_version = "latest"
-# makes default setting explicit:
-notfound_no_urls_prefix = False
+notfound_urls_prefix = "/ansible/latest/"
 
 # Options for LaTeX output
 # ------------------------