interventions: document holding all tasks, triggering and releasing

[oliver-sanders]

Document a common use case:

• Hold all tasks
• Trigger some tasks
• Release all tasks

Currently the solution is to use the "hold after cycle point" feature.

Note, when https://github.com/cylc/cylc-flow/issues/5827 is done, we will be able to simplify this to:

$ cylc hold 'workflow//*'
$ cylc trigger ...
$ cylc release 'workflow//*'

Requirements check-list

• [x] I have read CONTRIBUTING.md and added my name as a Code Contributor.

[oliver-sanders]

(merging with two approvals)