Summary

Resolves #19384.

• Distinguishes more clearly between date and datetime objects.
• Uniformly links to the relevant Python docs from rules in this category.

I've tried to be clearer, but there's still a contradiction in the rules as written: we say "use timezone-aware objects", but dates are inherently timezone-naive.

Also, the full docs don't always match the error message: for instance, in DTZ012, the example says to use:

datetime.datetime.fromtimestamp(946684800, tz=datetime.UTC)

while fix_title returns "Use datetime.datetime.fromtimestamp(ts, tz=...)**.date()** instead".
I have left this as it was for now.

Test Plan

Ran mkdocs locally and inspected result.