DOC: Add documentation for `groupby.expanding()`
- [x] closes #61254 
- [ ] ~[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).
- [ ] ~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.~

Thanks @arthurlw - If you're interested I also think it would be good to replace `*args` and `**kwargs` throughout the public functions in the pandas API with the exact arguments that the method takes. This is more user-friendly not only for docs, but for linting and auto-completion. Yeah I would be happy to work on replacing `*args` and `**kwargs` in public functions. I'll draft an issue and start listing down some of the public functions that need updating. Let me know if there’s a preferred place to start or if you’d prefer separate issues/PRs per module or function. I would recommend "one" method per PR (where e.g. Series.sum and DataFrame.sum counts as 1) but no issues need to be made unless it seems uncertain whether we want to replace *args / **kwargs. The one case where we don't want to replace them is when they are passed through to a third-party (e.g. matplotlib or xlsxwriter).