DOC: Add rolling in reference/groupby.rst
- [ ] close #51097



Since we have docstring checks, the docstrings of these will need to be improved in this PR

```
Error: /home/runner/work/pandas/pandas/pandas/core/groupby/groupby.py:2787:RT01:pandas.core.groupby.DataFrameGroupBy.rolling:No Returns section found
Error: /home/runner/work/pandas/pandas/pandas/core/groupby/groupby.py:2787:RT01:pandas.core.groupby.SeriesGroupBy.rolling:No Returns section found
Error: /home/runner/work/pandas/pandas/pandas/core/groupby/groupby.py:2787:EX01:pandas.core.groupby.DataFrameGroupBy.rolling:No examples section found
Error: /home/runner/work/pandas/pandas/pandas/core/groupby/groupby.py:2787:EX01:pandas.core.groupby.SeriesGroupBy.rolling:No examples section found
``` > Since we have docstring checks, the docstrings of these will need to be improved in this PR
> 
> ```
> Error: /home/runner/work/pandas/pandas/pandas/core/groupby/groupby.py:2787:RT01:pandas.core.groupby.DataFrameGroupBy.rolling:No Returns section found
> Error: /home/runner/work/pandas/pandas/pandas/core/groupby/groupby.py:2787:RT01:pandas.core.groupby.SeriesGroupBy.rolling:No Returns section found
> Error: /home/runner/work/pandas/pandas/pandas/core/groupby/groupby.py:2787:EX01:pandas.core.groupby.DataFrameGroupBy.rolling:No examples section found
> Error: /home/runner/work/pandas/pandas/pandas/core/groupby/groupby.py:2787:EX01:pandas.core.groupby.SeriesGroupBy.rolling:No examples section found
> ```

I added some docstring, but they definitely need further discussion. 

The parameters section is copied from [pandas.DataFrame.rolling](https://pandas.pydata.org/docs/dev/reference/api/pandas.DataFrame.rolling.html#pandas.DataFrame.rolling), the parameter `step` is not implemented, so I removed it.  Other parameters are implemented, but the exact description I'm not sure if I need to keep changing. Thanks @luke396 