Add KDocs for non-deprecated `sum` APIs

Add KDocs for non-deprecated sum APIs

The file core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/api/sum.kt holds the public sum APIs, and the coverage sheet marks the group sum/* as missing KDocs. None of these functions are currently documented (there is only a TODO KDocs comment block at the top of the file). Please add concise KDocs for the current non-deprecated APIs only, excluding deprecated and binary-compatibility (@AccessApiOverload, and the whole // region binary compatibility of DeprecationLevel.HIDDEN overloads) overloads.

Scope

APIs needing KDocs, by receiver:

• DataColumn.* — sum, sumOf
• DataRow.* — rowSum, rowSumOf
• DataFrame.* — sum, sumFor, sumOf
• Grouped.* — sum, sumFor, sumOf
• Pivot.* — sum, sumFor, sumOf
• PivotGroupBy.* — sum, sumFor, sumOf

KDocs should explain

• What each operation returns:
  • sum — the sum of the values (over the whole column / row / frame);
  • sumFor — a DataRow/DataFrame with a separate sum per selected column;
  • sumOf — the sum of a value computed per row/element via the given expression;
  • the grouped / pivoted variants — sum computed per group / per pivot cell.
• The common numeric behavior (already captured in the TODO KDocs block, lines 35–42):
  • supported for all primitive number types; for mixed primitive number types the values are unified before summing;
  • nulls are filtered out;
  • the return type is always the same as the input type and never null, except Byte and Short, which are returned as Int;
  • empty input yields 0 in the corresponding number type.
• The skipNaN parameter: how NaN values in floating-point columns are treated.
• The separate parameter for the Pivot / PivotGroupBy variants, and the optional name / resultName parameter for the Grouped variants.
• Note the rowSumOf overloads are split per concrete primitive type (Short/Byte/Int/Long/Float/Double) plus a KType overload, because a single reified T : Number? is not possible — a shared KDoc (e.g. via @include) is fine.

Acceptance criteria

• Concise KDocs are added to all non-deprecated sum / sumFor / sumOf / rowSum / rowSumOf overloads in sum.kt.
• No documentation is added for deprecated / @AccessApiOverload overloads, nor for anything in the // region binary compatibility block.
• KDocs cover the skipNaN parameter, the "same-as-input type, never null" return contract (with the Byte/Short → Int exception), empty-input → 0, and null-filtering.
• KDocs clearly distinguish the value-returning (sum, sumOf, rowSum*) variants from the per-column aggregation (sumFor) variants.
• Docs are consistent with the site docs at sum.html.
• The TODO KDocs comment block at the top of the file is removed once its content has been folded into the actual KDocs.