Add `PivotGroupBy` KDocs (reducing and aggregation methods)

### Problem

`PivotGroupBy` reducing and aggregation APIs currently lack KDocs.

This API is more specialized than `groupBy`/`pivot`, and its behavior (especially how reducing and aggregation work in pivot context) is not documented in the code.

Users cannot understand:
- how these operations differ from regular `groupBy`
- how results are structured
- what schema transformations happen

### Expected

Add KDocs for `PivotGroupBy` reducing and aggregation methods.

### Acceptance criteria

- KDocs are added for:
- `PivotGroupBy` reducing APIs
- `PivotGroupBy` aggregation APIs
- Each KDoc includes:
- description of behavior
- explanation of how pivoted groups are reduced/aggregated
- at least one usage example
- Relationship to `groupBy`/`pivot` APIs is clarified
- KDocs follow existing DataFrame API style

### Why it matters

- `PivotGroupBy` is part of advanced data transformation workflows
- Lack of documentation makes the API hard to understand and adopt
- Public API should be fully documented before 1.0 stabilization