DOC: Add pandas-stubs and potential 2.0 API breaks to 1.5 whatsnew
- [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.

* Added note about `pandas-stubs` library
* Added note about potential 2.0 API breaks: https://github.com/pandas-dev/pandas/issues/44823
> wouldn't be opposed to moving the pandas-stubs note up to the top or just under exchange protocol - I think this deserves visibility.

agreed.