Add KDocs for `reorder` APIs

## Add KDocs for `reorder` APIs

`core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/api/reorder.kt` contains public `reorder` APIs without KDocs.

The site page already documents the main behavior and should be used as the source of truth:
https://kotlin.github.io/dataframe/reorder.html

### Scope

Add concise KDocs for:

- `DataFrame.reorder { ... }`
- `DataFrame.reorder(vararg columns: String)`
- `Reorder.by { ... }`
- `Reorder.byDesc { ... }`
- `Reorder.byName(desc)`
- `DataFrame.reorderColumnsBy(...)`
- `DataFrame.reorderColumnsByName(...)`
- `Reorder.cast()`

Deprecated access-api overloads do not need new KDocs.

### KDocs should explain

- `reorder` returns a dataframe with selected columns reordered.
- Selected columns are reordered among their original positions; non-selected columns keep their positions.
- If selected columns belong to different column groups, they are reordered within their groups, preserving grouping.
- If exactly one `ColumnGroup` is selected, reordering is applied to its nested columns.
- `by` sorts selected columns by a column expression.
- `byDesc` applies descending order.
- `byName(desc)` sorts by column name.
- `reorderColumnsBy` reorders all columns, optionally recursively with `atAnyDepth`.
- `reorderColumnsByName` is a shortcut for name-based reordering.

### Acceptance criteria

- Public non-deprecated `reorder` APIs have concise KDocs.
- KDocs are consistent with the existing site page.
- Deprecated access-api overloads are skipped.
- KDocs cover subset behavior, column-group behavior, `desc`, and `atAnyDepth`.

Start in core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/api/reorder.kt and compare the public non-deprecated APIs with the reorder.html site page. Check each listed overload for coverage of subset and column-group behavior, desc, and atAnyDepth, while skipping deprecated access-api overloads. Done means concise KDocs are present and consistent with the site page.

Written by the indexing model from the issue text.