Bach/modelhub: improve documentation for setting up test databases

[thijs-obj]

Situation: Our functional tests run against Postgres, BigQuery, and Athena. We have setup those databases so we can just run the tests. Anyone forking our repository would need to do the same setup before they can run all tests themselves. Ideally we'd clearly document how to set that up

Goal: document in one place how to setup databases for functional tests. Ideally something that's very easily copy pasteable or even executable.

Discussion: https://github.com/objectiv/objectiv-analytics/pull/1112#discussion_r937725117