DOC: Add documentation for `groupby.ewm()`

[arthurlw]

Pandas version checks

• I have checked that the issue still exists on the latest versions of the docs on main here

Location of the documentation

https://pandas.pydata.org/docs/dev/reference/groupby.html

Documentation problem

There is no reference for DataFrameGroupBy.ewm(), even though it exists in API, and Docstring can be greatly improved.

Similar to: #61254

E.g. consider working example:

>>> import pandas as pd
>>> pd.__version__
'2.2.3'
>>> data = {"Class": ["A", "A", "A", "B", "B", "B"],"Value": [10, 20, 30, 40, 50, 60],}
>>> df = pd.DataFrame(data)
>>> df
  Class  Value
0     A     10
1     A     20
2     A     30
3     B     40
4     B     50
5     B     60
>>> ewm_mean = (df.groupby("Class").ewm(span=2).mean().reset_index(drop=True))
>>> ewm_mean
   Value
0  10.000000
1  17.500000
2  26.153846
3  40.000000
4  47.500000
5  56.153846

Suggested fix for documentation

Include reference of DataFrameGroupBy.ewm and SeriesGroupBy.ewm, like for DataFrameGroupBy.rolling
and SeriesGroupBy.rolling

Improve groupby.ewm() function Docstring

[arthurlw]

take