Add KDocs for `forEach` API

`core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/api/forEach.kt` has public API functions without KDocs.

## Motivation

`forEach` APIs look simple, but receiver-specific behavior matters: `DataColumn`, `DataFrame`, and `GroupBy` iterate over different things. Concise KDocs would improve IDE discoverability and help LLM agents understand the intended iteration model directly from the code.

## Scope

Add concise KDocs for all public declarations in `forEach.kt`:

- `DataColumn.forEach`
- `DataColumn.forEachIndexed`
- `DataFrame.forEach`
- `GroupBy.forEach`

The docs should clarify:

- What is iterated over for each receiver.
- What arguments are passed to the callback.
- That these functions are side-effect-oriented and return `Unit`.

## Acceptance Criteria

- All public declarations in `forEach.kt` have useful KDocs.
- KDocs are concise and consistent with the existing API style.
- Documentation generation succeeds.

Open core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/api/forEach.kt and review the four listed public declarations alongside nearby API documentation for style. Add concise KDocs describing each receiver’s iteration target, callback arguments, and Unit return, then run the project’s documentation generation to verify it succeeds.

Written by the indexing model from the issue text.