Document how to add required drivers for databases

[maltesander]

Superset supports a long list of databases https://superset.apache.org/docs/configuration/databases/, most of them requiring additional drivers to install.

We kept away from including different drivers in our images due to licensing.

However, we should document how you could use theses databases / drivers if required.

Possible options:

• Build your own docker image containing the required driver based on the Stackable docker image https://github.com/stackabletech/docker-images/blob/main/superset/Dockerfile
• Throw the driver into a volume and mount that into superset