[`flake8-datetimez`] Clarify docs for several rules (#20778)
## 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 `date`s are
inherently timezone-naive.
Also, the full docs don't always match the error message: for instance,
in [DTZ012](https://docs.astral.sh/ruff/rules/call-date-fromtimestamp/),
the example says to use:
```python
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.
