Add section to changes on triggering tasks at icp and fcp using regex…

[wxtim]

… inspired syntax

Requirements check-list

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

[oliver-sanders]

Err to fix:

/home/runner/work/cylc-doc/cylc-doc/src/reference/changes.rst:116: WARNING: Inline literal start-string without end-string. [docutils]
/home/runner/work/cylc-doc/cylc-doc/src/reference/changes.rst:116: WARNING: Inline interpreted text or phrase reference start-string without end-string. [docutils]

[oliver-sanders]

Ach, the code block is in markdown syntax! That'll do it.

[wxtim]

Ach, the code block is in markdown syntax! That'll do it.

• RST 🎉
• Markdown 👍🏼
• Switching 🤮