[Task]: Add more info on setting up alloy db instance in notebook

[damccorm]

What needs to happen?

I was going through https://cloud.google.com/dataflow/docs/notebooks/alloydb_product_catalog_embeddings and couldn't quite get my DB set up correctly - I was getting:

An SQLAlchemy error occurred during setup: (pg8000.exceptions.DatabaseError) {'S': 'FATAL', 'V': 'FATAL', 'C': '3D000', 'M': 'database "dannytestdbfordemo" does not exist', 'F': 'postinit.c', 'L': '1148', 'R': 'InitPostgres'}
(Background on this error at: https://sqlalche.me/e/20/4xp6)

I'm not 100% sure what was wrong, but I think we would benefit from having a little more info on setting up the alloy instance in the prerequisites (or if we can script this, it would be even better).

Issue Priority

Priority: 2 (default / most normal work should be filed as P2)

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