Poor documentatio for MySQL and MariaDB connection

[tytuseczek]

Hi.

I am struggling to make a preset connection in config.py to mariadb or Mysql database.

Page https://omnidb.readthedocs.io/en/2.17.0/en/19_deploying_omnidb-server.html#configuration-file should be updated.

DATABASES section should be described and different supported engines should be described.
As I see in example there are more other features supported in config file that are not in the docs, please update.