Add `PivotGroupBy` KDocs (reducing and aggregation methods)

[AndreiKingsley]

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

[zaleslaw]

@AndreiKingsley Please start with that