uis: auto-document Traitlets configurations

[oliver-sanders]

The UIS now has some configurations.

We should be able to auto-document these, see https://traitlets.readthedocs.io/en/stable/config-api.html#traitlets.config.Application.document_config_options

[oliver-sanders]

Easy enough to do, only issue is that it only documents application defaults as opposed to traitlet defaults which means it gets the defaults all wrong which is highly confusing.

Blocked pending investigation in cylc-uiserver to see if we can move the defaults (of configurable=True traits only) out of @default methods into the traits (via the default kwarg).