examples: document expiry workflow design patterns

[oliver-sanders]

Companion: cylc/cylc-doc#796

Document workflow design patterns using automated task expiry.

I put together a couple of examples for user support today and realised the expiry docs were a bit lacking.

These will now be documented workflow design patterns with functional tests.

Check List

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Applied any dependency changes to both setup.cfg (and conda-environment.yml if present).
• Tests are included (or explain why tests are not needed).
• Changelog entry included if this is a change that can affect users
• Cylc-Doc pull request opened if required at cylc/cylc-doc/pull/XXXX.
• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.

[hjoliver]

Good!

[hjoliver]

I'll merge this - one approval, @markgrahamdawson 's typo-fix was applied, and it's docs-only.