DOC: add documentation to DataFrameGroupBy.skew and SeriesGroupBy.skew
- [ ] 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.

Contributes to https://github.com/noatamir/pyladies-berlin-sprints/issues/13

I copied the documentation from [here](https://github.com/pandas-dev/pandas/blob/main/pandas/core/generic.py#L11496) and [here](https://github.com/pandas-dev/pandas/blob/main/pandas/core/generic.py#L11825). Let me know if I should delete the lines in SeriesGroupBy.skew that apply to DataFrames and vice versa. 

ping @phofl @noatamir 

#PyLadies Berlin sprint

I made the requested changes. I will work on the CI failures tomorrow.  Thanks @marenwestermann - great work! Thx @marenwestermann and thx @rhshadrach for reviewing Thank you and thanks for reviewing! 