Add recommend migration tools for Postgres

[exaby73]

Since alterTable doesn't work with Postgres, I'd want an automatic migration tool that can handle this with Postgres. I'm not sure if the schema exports that Drift generates via make-migrations are of any standard that other tools would understand, which is why I'd like the docs to give some pathways on how we can get similar devx with Postgres and 3rd party migration tools that work well with Drift

[simolus3]

It's worth noting that alterTable is sort of a catch-all commonly used with SQLite because most advanced ALTER statements aren't supported. Postgres has supporting for changing column types or constraints, for instance.

But since we already recommend an external migration tool for now, I agree that we should document what's available and how it can be used with drift.