DOC/ENH: Add documentation and whatsnew entry for ArrowDtype and ArrowExtensionArray
- [x] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [x] Added an entry in the latest `doc/source/whatsnew/v1.5.0.rst` file if fixing a bug or adding a new feature.

* Depends on https://github.com/pandas-dev/pandas/pull/47818
* Should I add a dedicated user-guide page for this functionality?
Will merge tomorrow if no further comments.