[docs] Migrate Commit_cheat_sheet to general/development/abc/commit-comments.md and other places

[timhunt]

The URL of the page on https://docs.moodle.org/dev/

https://docs.moodle.org/dev/Commit_cheat_sheet

Is this documentation specific to a Moodle version?

No

What location would you suggest in the new docs?

general/development/abc/commit-comments.md

Note, work started at timhunt@commits, but needs a bit of refactoring before I am ready to open a pull request.