Add KDocs for groupBy and pivot APIs (core DSL + aggregation)

### Problem

`groupBy` and `pivot` are core APIs with complex DSL, but KDocs are missing or incomplete.

This makes API hard to understand without external documentation.

### Scope

Cover KDocs for:
- `groupBy` entry points
- `pivot` entry points
- core DSL blocks (`aggregate`, `reduce`, etc.)

(Subtasks track specific functions)

### Acceptance criteria

- KDocs added for all public `groupBy` and `pivot` entry points
- KDocs include at least:
- short description
- one minimal example
- Core DSL concepts (`aggregate`, `reduce`) are explained
- All subtasks are completed

### Motivation

`groupBy`/`pivot` are among the most complex and important APIs.

They must be understandable from IDE before 1.0 without relying on external docs.

Related to #811

The [`groupBy` operation](https://kotlin.github.io/dataframe/groupby.html) is one of the most advanced operations in DataFrame. It has an extensive DSL that is only documented on the website at the moment. It's a hard task but one we must do eventually.

The same holds for the [`pivot` operation](https://kotlin.github.io/dataframe/pivot.html). Let's try to make people not scared of this advanced function anymore :)

I've grouped them together in one issue because they call into each other's DSL. This needs to be shown in the KDocs and the grammar DSLs.

Take note of `aggregate` and all the functions inside it too. This could link to a new common "statistics" definition.
A common `default()` definition could also likely be created :)