DOC: Fix docstrings with the sections in the wrong order

[datapythonista]

After #24132 is merged, we'll have few docstrings where the order of the sections is wrong:

pandas.Series.agg: Sections are in the wrong order. Correct order is: Parameters, Returns, See Also, Notes, Examples
pandas.Series.aggregate: Sections are in the wrong order. Correct order is: Parameters, Returns, See Also, Notes, Examples
pandas.DataFrame.agg: Sections are in the wrong order. Correct order is: Parameters, Returns, See Also, Notes, Examples
pandas.DataFrame.aggregate: Sections are in the wrong order. Correct order is: Parameters, Returns, See Also, Notes, Examples
pandas.core.window.Rolling.aggregate: Sections are in the wrong order. Correct order is: Parameters, Returns, See Also, Notes, Examples
pandas.core.window.Expanding.aggregate: Sections are in the wrong order. Correct order is: Parameters, Returns, See Also, Notes, Examples
pandas.core.groupby.GroupBy.head: Sections are in the wrong order. Correct order is: See Also, Examples
pandas.core.groupby.GroupBy.mean: Sections are in the wrong order. Correct order is: Returns, See Also, Examples
pandas.core.groupby.GroupBy.nth: Sections are in the wrong order. Correct order is: Parameters, See Also, Examples
pandas.core.groupby.GroupBy.tail: Sections are in the wrong order. Correct order is: See Also, Examples
pandas.core.groupby.DataFrameGroupBy.agg: Sections are in the wrong order. Correct order is: Parameters, Returns, See Also, Notes, Examples
pandas.core.resample.Resampler.apply: Sections are in the wrong order. Correct order is: Parameters, Returns, See Also, Notes, Examples
pandas.core.resample.Resampler.aggregate: Sections are in the wrong order. Correct order is: Parameters, Returns, See Also, Notes, Examples
pandas.core.resample.Resampler.mean: Sections are in the wrong order. Correct order is: Returns, See Also, Examples

We should fix the order of those, and add to ci/code_checks.py, in the part where scripts/validate_docstrings.py is called, the error GL07, so when docstrings are created or modified, the CI validates that all the pandas docstrings still have the right order.

The next command can be used to see if all the cases are fixed: $ ./scripts/validate_docstrings.py --errors=GL07, but before #24132 is merged, it will also return the cases already fixed in theat PR.

[benjaminr]

Happy to have a look at this one.

[datapythonista]

thanks @benjaminr

as it says in the description, make sure you just fixed the ones listed, otherwise you'll get conflicts when we merge #24132