Add KDocs for non-deprecated `toDataFrame` / `toDataFrameFromPairs` APIs

Add KDocs for non-deprecated toDataFrame / toDataFrameFromPairs APIs

The file core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/api/toDataFrame.kt holds the public toDataFrame / toDataFrameFromPairs conversion APIs and their supporting DSL. It contains many overloads; almost none are documented (only List<List<T>>.toDataFrame(header, containsColumns) and a few TraversePropertiesDsl members have KDocs).

Please add concise KDocs for the current non-deprecated APIs only. Deprecated and binary-compatibility (@AccessApiOverload) overloads are out of scope.

Scope — main semantic blocks

• From objects (reflection-based) — Iterable<T>.toDataFrame() and its variants (the CreateDataFrameDsl body overload, the vararg props / maxDepth overload, and the single-columnName overload). These turn each object into a row by reading its properties.
• From rows / columns / maps — Iterable<DataRow>, Iterable<BaseColumn>, Iterable<Map<String, Any?>>, and the Map<String, Iterable> / Map<ColumnPath, Iterable> overloads.
• From pairs (toDataFrameFromPairs) — the ColumnPath/String → column/values overloads that build (possibly nested) column groups from path→column pairs.
• From lists of lists — List<List<T>>.toDataFrame(header, containsColumns) (already documented; keep/align).
• The building DSL — the CreateDataFrameDsl class and its members (add, expr, into, from, inferType, String.invoke, properties), the TraversePropertiesDsl (exclude / preserve, partly documented already), and the ValueProperty schema interface.

KDocs should explain

• At the block level, what the conversion produces: how source elements map to rows vs columns, and how nested structures become column groups (especially for the property-reflection and toDataFrameFromPairs blocks).
• For the reflection block: that columns are derived from properties, the meaning of maxDepth (recursion depth for nested objects) and props (explicit property roots), and how the CreateDataFrameDsl lets users add/rename/compute columns.
• For TraversePropertiesDsl: exclude (skip classes/properties during traversal) and preserve (keep a type as a ValueColumn instead of expanding it) — extend the existing short KDocs into full ones.
• For toDataFrameFromPairs: how column paths with size > 1 are grouped into nested ColumnGroups, and that duplicate top-level names are made unique.
• Column name generation / uniqueness and type inference (createByInference / createColumnGuessingType) where relevant.
• The difference between toDataFrame and toDataFrameFromPairs (the latter takes explicit path→column pairs and is the nesting-aware builder).

Acceptance criteria

• Every public, non-deprecated toDataFrame / toDataFrameFromPairs overload is covered by a KDoc, grouped by the semantic blocks above.
• The CreateDataFrameDsl, TraversePropertiesDsl, and ValueProperty public types and their non-deprecated members are documented; the existing exclude / preserve / List<List<T>>.toDataFrame KDocs are kept or expanded, not removed.
• No documentation is added for deprecated / @AccessApiOverload overloads.
• KDocs make clear how source shapes map to rows/columns and how nesting (column groups) is produced.
• Docs are consistent with the site docs at createDataFrame.html.

Related to the #1479

Start with core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/api/toDataFrame.kt and compare its public non-deprecated APIs with the createDataFrame.html site documentation. Review the existing KDocs and semantic blocks, then document the specified overloads and DSL members, including source-to-row/column mapping, nesting, inference, uniqueness, and traversal behavior without covering deprecated or @AccessApiOverload APIs.