Golden source of truth for init scripts needed

[jcoresongres]

Bug report

Describe the bug

For self-hosting a Postgres database, we'd need a golden source of truth for the init scripts, that is a bit clearer documented than what is now pointed to by the documentation (https://github.com/supabase/postgres/tree/develop/migrations).

The issue is that some (?) of the Supabase containers seem to have their own migrations and init scripts that they run on top of the created initial schema, thus restricting the database user permissions didn't work for now. Ideally, there would be a setup where all Supabase-required database resources are set up before (in a single script run), and the Supabase containers just connect to the existing setup and need no additional work besides what is required in a project.