Document how to delete all records from all tables

[molebox]

This PR documents how to delete all records from all tables using postgres, sqlite and mysql. It gives an example for doing this with each database. The info is added to the CRUD page. Though this page might get split up into sections by operation later.

Fixes #451

[mhwelander]

One other option that is already documented - use deleteMany in a particular order, run all queries in a transaction: https://www.prisma.io/docs/guides/prisma-guides/prisma-client-transactions-guide#scenario-privacy-legislation