Improve Django ORM docs (best practices)

[ericharmeling]

Eric Harmeling (ericharmeling) commented:

Follow-up to #6359 and #6365.

We should have some best practices for using this ORM, like we've done for https://www.cockroachlabs.com/docs/stable/build-a-python-app-with-cockroachdb-sqlalchemy.html#best-practices.

Jira Issue: DOC-415