jupyter: document how to add Cylc to an existing setup

[oliver-sanders]

Some sites will want to deploy Cylc UI Server along with other Jupyter Server extensions such as Jupyter Lab.

These sites might not use the cylc hub wrapper as they might have an existing Jupyter Hub setup and might not want Cylc to be the default endpoint.

We could do with some better documentation to assist with this:

• Document how to change the default extension endpoint (i.e. which app gets loaded by default).
• Document how to add Cylc UI Server to an existing Jupyter Server deployment.
• Document how to configure and existing Jupyter Hub deployment to add Cylc.