Docs: add clear postgresql user/db setup instructions.

[brabo]

If a new Django user is not familiar with using postgresql yet, they may find various instructions online to setup the database and user. Incorrectly setting permission in the psql shell may turn into a rabbit hole with irrelevant "fix" after "fix" being applied until it somehow Just Works TM.
IMHO it would be really nice to add basic instructions on how to setup a user and db that will work with the tutorial and/or provide a sane starting point for a new project.
I am willing to provide a PR for this issue, but welcome any comments/thoughts/ideas anyone has about my opinion in this regard, and/or how to go about adding this info to the current documentation.

[Tim Graham]

Thanks for the suggestion, however, it's out of scope for Django's documentation. Django has several built-in database backends, and maintaining instructions for different database versions, operating systems, etc. quickly turns into a complicated task. Perhaps you might suggest an improvement to PostgresSQL's documentation if you found it lacking.

[brabo]

My bad. Seeing specific mysql instructions(​https://docs.djangoproject.com/en/5.2/ref/databases/#mysql-notes) and lacking instructions for postgresql made me think a simple PR to add a few lines would be appreciated.