DOC: fix RT03, ES01 for pandas.core.groupby.SeriesGroupBy.agg and pandas.core.groupby.SeriesGroupBy.aggregate (#59898)

* DOC: fix RT03, ES01 for pandas.core.groupby.SeriesGroupBy.agg

* DOC: remove _agg_template_series

Co-authored-by: mroeschke <mroeschke@users.noreply.github.com>
Co-authored-by: rhshadrach <rhshadrach@users.noreply.github.com>

* DOC: remove _agg_template_series

Co-authored-by: mroeschke <mroeschke@users.noreply.github.com>
Co-authored-by: rhshadrach <rhshadrach@users.noreply.github.com>

* DOC: remove _agg_template_seris

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Co-authored-by: mroeschke <mroeschke@users.noreply.github.com>
Co-authored-by: rhshadrach <rhshadrach@users.noreply.github.com>