Review setting up Sourcify DB docs

[kuzdogan]

As a general comment, I had some difficulty setting up the Sourcify database backend. As someone who isn't all that familiar with Postgres, it took me a while to figure out I had to set up the user, database, and change the authentication for the public schema myself. Some steps for doing this setup before running the migration scripts would be nice to have in the docs (though this was for a fresh instance and didn't do any actual migration).

Quoting from the feedback from the public chat, we should review how to set up the DB from scratch, and potentially modify the migrations to set everything up.

[manuelwedler]

Related: #1981