Add `on_delete = "Cascade"` to an example in the docs

Motivation

I was confused on how to add an ON DELETE CASCADE constraint via macros. The docs mention it once, a generic example randomly includes it, but the deletion page never talks about it.

Proposed Solutions

It would be great if the deletion page could include an example of a more complex case with relations. A few words about the difference between adding on_delete = "Cascade" to the relation and calling .cascade_delete(). (I noticed that the later didn't work on its own?)

Additional Information

Start with the documentation's deletion page and find the existing on_delete example and any references to cascade_delete(). Add a relations example showing on_delete = "Cascade" and explain how it differs from calling .cascade_delete(). Done means the deletion page clearly covers both approaches and the example renders correctly.