DOC: use Hashable instead of label
In https://github.com/pandas-dev/pandas/pull/61455#discussion_r2096069007:
> we should actually be using `Hashable` everywhere as since it's an actual Python type unlike `label`.

- <del>[ ] closes #xxxx (Replace xxxx with the GitHub issue number)</del>
- <del>[ ] [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).
- <del>[ ] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.</del>
- <del>[ ] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.</del>

Thanks @cmp0xff 