Document public events in repo dev docs #4179

Create a new dev docs file to document & detail all public JS-based events, such as the editor events.
Just so we have a document attached to code for developers to use, and a place to provide more detail to help users.

Would be good to document an event(/property?)-naming scheme.

Useful scraps

• See CM6 theme example here: Upgrade to codemirror 6 #3617 (comment)

Docs changes

• Should update "Hacking BookStack > Editor Events" docs section to be higher-level and point to this new doc.