[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