Document the Cylc UIs

[hjoliver]

We don't seem to have a specific documentation section on the available Cylc UIs, other than this, in the Cylc 8 migration guide which doesn't mention the CLI.

Quite important and quite easy to do:

• CLI: document that all things can be done in the terminal; advise cylc help all to see all the commands
• TUI and GUI - just take and update the migration guide docs?

[hjoliver]

(Posted after user query - apparently cylc help all isn't obvious to all, and it isn't documented).