user-guide: add interventions section

[oliver-sanders]

• Closes https://github.com/cylc/cylc-doc/issues/626
• Attempt to document the most common operator interventions.
• Add the sphinx-design library to the docs environment for tabbed selectors.

Once the skip-mode and remove features are implemented, we should extend this.

Requirements check-list

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

[oliver-sanders]

Build error should be fixed by: https://github.com/cylc/cylc-flow/pull/6087

[hjoliver]

(local build successful)