DOC: Add release notes for 1.5.1
- [ ] closes #xxxx (Replace xxxx with the Github issue number)
- [ ] [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
- [ ] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [ ] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [ ] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.


cc @datapythonista can we push stuff again and create a 1.5.1 milestone? 

The release is not yet done, but I don't see a reason not to push to main. As far as I can tell only the release candidate was tricky, since it's when we branch from main. Also ok to backport to `1.5.x`, we already got the tag in place, so I guess any changes after the tag will be ignored in the conda-forge and macpython builds.

I created the 1.5.1 milestone and move all 1.5 PRs to it. Thanks very much. Can move whatsnews over to then after this is merged. Great!