phd: improve setup documentation
Summary:
Address some concerns raised on D26181 after it landed.
- Running mysql on the same host: provide guidance on adding a dependency on mariadb.
- Linking directly: clarify that users can copy the unit, but now with `cp` instead of `ctrl+c`.
- User/group need changing: provide instructions on configuring the unit with systemd "drop-ins".
Test Plan:
- Check over the prose to make sure it's not too confusing.
- Confirm that the commands given to users do what they're supposed to do.
- Run `./bin/diviner generate` and view the results to ensure they're visually pleasing.
Reviewers: avivey, O1 Blessed Committers
Reviewed By: avivey, O1 Blessed Committers
Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Differential Revision: https://we.phorge.it/D26215