REF: Prefer testing and documenting zoneinfo instead of pytz
Split off from https://github.com/pandas-dev/pandas/pull/57045

* Add more documentation about `zoneinfo.ZoneInfo` objects being accepted
* Changed tests/benchmark to prefer stdlib timezone objects instead of pytz objects e.g. `pytz.utc` -> `datetime.timezone.utc`, `pytz.timezone("US/Pacific")` -> `zoneinfo.ZoneInfo("US/Pacific")`
Going to merge this PR to make progress on https://github.com/pandas-dev/pandas/pull/57045, happy to follow up if needed