Version 3 documentation

I’m a little behind on documentation for version 3. I’ve started on it, feel free to submit any PR for anything that needs to be added.

https://bluecherry-apps.readthedocs.io/en/v3-docs/

FYI, this link throws an error that this page does not exist.

Sorry, the correct link for the latest documentation is Table of Contents — Bluecherry 2.8 documentation