DOC: change `pandas.DataFrame.unstack`'s `fill_value` param to scalar
- [x] closes #61445 (Replace xxxx with the GitHub issue number)
- [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 [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [x] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

<img width="796" alt="image" src="https://github.com/user-attachments/assets/2540c012-dc39-47bd-81f9-263855a2c69d" />

Thanks @KevsterAmp 