Add KDocs for `valueCounts` APIs

[zaleslaw]

Add KDocs for valueCounts APIs

The file core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/api/valueCounts.kt has no KDocs at all. Please add concise KDocs for the non-deprecated APIs, excluding deprecated / @AccessApiOverload overloads.

Scope

Non-deprecated APIs in valueCounts.kt:

• The ValueCount @DataSchema interface (count: Int) — the result schema.
• DataColumn<T>.valueCounts(sort, ascending, dropNA, resultColumn) — returns a DataFrame<ValueCount>.
• DataFrame<T>.valueCounts(sort, ascending, dropNA, resultColumn, columns) — selector overload.
• DataFrame<T>.valueCounts(vararg columns: String, sort, ascending, dropNA, resultColumn) — by-name overload.

Out of scope: the deprecated vararg columns: AnyColumnReference / vararg columns: KProperty<*> overloads.

KDocs should explain

• What valueCounts does: counts the occurrences of each distinct value (for a column) or each distinct row / combination of the selected columns (for a DataFrame), returning a new frame with the distinct values plus a count column.
• The parameters and their defaults:
  • sort (default true) — whether to sort by count;
  • ascending (default false) — sort direction, i.e. by default most frequent first;
  • dropNA (default true) — whether NA/null values are excluded from counting;
  • resultColumn (default "count") — the name of the count column, and that it is made unique if it clashes with an existing column name (e.g. count → count1);
  • columns — which columns to count distinct combinations of (whole frame if null).
• The result shape: the value/selected columns are preserved, with the added count column of type Int; the DataFrame result schema is ValueCount-based.

Test coverage assessment

• There is no dedicated test file for valueCounts.kt; it is only exercised incidentally (samples/api/Analyze.kt, testSets/person/DataColumnTests.kt).
• Optional follow-up: a focused test file covering sort/ascending, dropNA behavior with nulls, the resultColumn name-clash handling, and multi-column counting.

Website coverage

• Documented at valueCounts.html; keep KDocs consistent with it.

Acceptance criteria

• All non-deprecated valueCounts overloads and the ValueCount schema interface have concise KDocs.
• No documentation is added for the deprecated / @AccessApiOverload overloads.
• KDocs cover every parameter and its default, the dropNA and count-column-name-clash behavior, and the result shape.
• Docs are consistent with the site page valueCounts.html.