doc: link workflow event configs to user guide section

[oliver-sanders]

Sibling: cylc/cylc-doc#685

Adds a link to the workflow events section of the user guide for each workflow event.

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).
• CHANGES.md entry included if this is a change that can affect users
• Cylc-Doc pull request opened if required at improve workflow events documentation cylc-doc#685.
• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.

[MetRonnie]

Waiting on merge of cylc/cylc-doc#685