cylc hub: document multi-user config post jupyter server v2 upgrade

* Auto-spawning of other user's servers can now be configured.
* Jupyter Server security policy must be defined if Jupyter Lab
  or any other server extension is included in the deployment.
* Simplify some sections and expand on the new configurations.

src/conf.py

@@ -110,6 +110,8 @@
 autosummary_imported_members = False
 
 # Mapping to other Sphinx projects we want to import references from.
+# NOTE: To work search available references, use:
+# $ python -m sphinx.ext.intersphinx <url>/objects.inv | less
 intersphinx_mapping = {
     'rose': (
         'http://metomi.github.io/rose/2.0.0/html', None
@@ -119,6 +121,9 @@
     ),
     'jupyter_server': (
         'https://jupyter-server.readthedocs.io/en/latest/', None
+    ),
+    'jupyter_hub': (
+        'https://jupyterhub.readthedocs.io/en/stable/', None
     )
 }