DOC: Added array section for DatetimeArray

[TomAugspurger]

Followup to #23581, blocked by #24024.

Need to add a section to api.rst about DatetimeArray, TimedeltaArray, and DatetimeTZDtype.

[TomAugspurger]

FYI, this needn't block the release. It can be done during the RC if #24626 isn't done in time.