DOC: Add documentation for groupby.expanding()

[arthurlw]

• closes DOC: Add documentation for groupby.expanding() #61254
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations 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.

[rhshadrach]

Thanks for the PR!

[rhshadrach]

lgtm

[rhshadrach]

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.

[arthurlw]

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.

[rhshadrach]

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).