Add `groupBy` and `pivot` reducing KDocs

[AndreiKingsley]

Problem

groupBy and pivot reducing operations currently lack KDocs.

These operations define how grouped data is reduced (e.g. aggregations, combining values), but their behavior is not documented in the API itself.

Users must rely on external documentation or guess semantics.

Expected

Add KDocs for all reducing-related groupBy and pivot operations.

Acceptance criteria

• KDocs are added for:
  • groupBy reducing APIs
  • pivot reducing APIs
• Each KDoc includes:
  • description of reduction behavior
  • explanation of how results are produced from groups
  • at least one usage example
• Differences between reducing and transformation operations are clarified
• KDocs follow existing style of other DataFrame APIs

Why it matters

• Reducing operations are core to data aggregation workflows
• Lack of documentation makes behavior unclear and error-prone
• Public API should be fully documented before 1.0 stabilization

[zaleslaw]

@AndreiKingsley Please start with that