Add KDocs for pivot `with` APIs

[zaleslaw]

Add KDocs for pivot with APIs

The file core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/api/with.kt has no KDocs at all. Please add concise KDocs for all four functions.

Scope

All four are the pivot aggregation with(expression):

• Pivot<T>.with(expression: RowExpression<T, V>): DataRow<T>
• ReducedPivot<T>.with(expression: RowExpression<T, V>): DataRow<T>
• PivotGroupBy<T>.with(expression: RowExpression<T, V>): DataFrame<T>
• ReducedPivotGroupBy<T>.with(expression: RowExpression<T, V>): DataFrame<T>

KDocs should explain

• What with does: it is a pivot aggregation terminal that fills each pivot cell with the value produced by evaluating expression on the group's rows — the result value type V is captured via the reified type argument.
• The receiver differences and result shape:
  • Pivot / ReducedPivot produce a single DataRow<T> (one value per pivot column);
  • PivotGroupBy / ReducedPivotGroupBy produce a DataFrame<T> (a value per group × pivot column);
  • the Reduced* variants evaluate the expression on the single reduced row of each group/cell, the non-reduced variants over the whole group.
• The expression parameter: a RowExpression<T, V> evaluated per group; note (for ReducedPivotGroupBy) that returning a column reference resolves to that column's value.

Test coverage assessment

• There is no dedicated test file for with.kt. Optional follow-up: cover each receiver variant and the reduced-vs-non-reduced behavior.

Website coverage

• with is covered on pivot.html (as a pivot aggregation step); keep KDocs consistent with it.

Acceptance criteria

• All four with overloads have concise KDocs.
• KDocs state the pivot-aggregation semantics, the DataRow vs DataFrame result per receiver, and the reduced-vs-non-reduced distinction.
• Docs are consistent with the site page pivot.html.