Add small documentation details

Summary:
Add some small Diviner documentation details like:

• mention PHP 8.1 (thanks https://we.phorge.it/Q18)
• mention the fact that you need multiple databases (it may be not obvious from the startup)
• mention MariaDB and not just MySQL (thanks MariaDB community!)
• mention "Administrator" and not just "User" for the "Instructions for installing, configuring, and using Phorge."

I was not bold enough to do more changes.

Test Plan: I tested locally and Diviner does not look bad.

Reviewers: O1 Blessed Committers, avivey

Reviewed By: O1 Blessed Committers, avivey

Subscribers: avivey, speck, tobiaswiese, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25064