refactor: Light editing of server tuning guide

Mostly just typos, minor formatting matters, etc.
Minor rewording in a couple spots.

Other notable changes:

- Renamed *Using MariaDB/MySQL instead of SQLite* section to *Replacing SQLite* and expanded it to incorporate PostgreSQL as a coequal option
- Added *Tuning your database* section which was really a place to move the MariaDB tuning bit that was in the previous section. Also added a PostgreSQL relevant link there for consistency.

Signed-off-by: Josh <josh.t.richards@gmail.com>

Author: joshtrichards