example: extending workflow

[oliver-sanders]

Important

This is #5947 with the "complex" example (to which we do not currently have a generic solution) chopped of. The remainder of that PR will need to wait, however there is pressure for us to get the simple bit documented now.

Document the preferred approach to "extending" a workflow.

Partially addresses: cylc/cylc-doc#525
Partially addresses: #5875

Context:

A common pattern in some areas is to run a workflow until it stops. Then modify the final cycle point to extend the workflow and restart.

This isn't an easy working pattern to port to Cylc 8 due to the difficulty of re-populating the start tasks. We have a plan to make this easier, see #5416, however, this working pattern is needlessly complex in the first place. It would be much simpler if the workflow didn't "finish" in the first place.

To do this, swap final cycle point for stop after cycle point, now the workflow will not "finish" so the pool will not need re-polulating for the workflow to continue. This is much nicer for Cylc (no discontinuity), the user (no extra commands) and provenance (direct continuation of previous run).

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.