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GH-34579: [Python][Docs] TableGroupBy.aggregate options (#34759)

### Rationale for this change

Add more information and examples to `pa.TableGroupBy.aggregate` method to make it clearer to use.

### What changes are included in this PR?

Changes in the `pa.TableGroupBy.aggregate` docstrings include:
- link to https://arrow.apache.org/docs/python/compute.html#grouped-aggregations
- extra examples
* Closes: #34579

Lead-authored-by: Alenka Frim <frim.alenka@gmail.com>
Co-authored-by: Alenka Frim <AlenkaF@users.noreply.github.com>
Co-authored-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>
Signed-off-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>