[docs] Switch from MySQL to MariaDB

Summary: This updates our dev environment instructions to tell the user to install MariaDB instead of MySQL.

Test Plan: I ran these instructions on my own machine and confirmed things work. I also created this Gist with instructions for migrating

Reviewers: atul, tomek

Reviewed By: atul

Subscribers: adrian, abosh

Differential Revision: https://phab.comm.dev/D4578