Add KDocs for `DataFrameGet` accessors

[zaleslaw]

Add KDocs for DataFrameGet accessors

The file core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/api/DataFrameGet.kt is almost entirely undocumented — only properties() has a KDoc. Add concise KDocs for the non-deprecated get* accessors, excluding deprecated / @AccessApiOverload overloads.

Already documented (leave as-is)

• DataFrame<T>.properties().

Scope — not documented (grouped)

• Paths & columns-with-paths (by selector) — getColumnsWithPaths, getColumnPath, getColumnPaths, getColumnWithPath.
• Get columns — getColumns(selector), getColumns(vararg String), getColumnIndex(col).
• Get a single column (getColumn family) — by name / ColumnPath / index / selector, plus the typed getColumnGroup (index/name/selector), getFrameColumn (path/name), and the *OrNull variant getColumnGroupOrNull(name).
• Get rows — getRows(range), getRows(indices), getOrNull(index), rows(), rowsReversed().

Out of scope: all deprecated ColumnReference / KProperty overloads of getColumn / getColumnGroup / getColumnGroupOrNull / containsColumn / contains.

KDocs should explain

• What each accessor returns and how its argument is interpreted (by name vs ColumnPath vs index vs selector) — this is what disambiguates the many same-named overloads.
• Failure vs null behavior: the getColumn*/getFrameColumn/getColumnGroup accessors throw IllegalArgumentException when not found (with the existing precise messages), while getColumnGroupOrNull / getOrNull return null; getOrNull(index) returns null for out-of-range/negative indices.
• The .single() accessors (getColumnPath, getColumnWithPath): they expect the selector to resolve to exactly one column and throw otherwise.
• rows() vs rowsReversed(): lazy forward/reverse iteration over DataRows.

Recommendations — make these KDocs genuinely useful (for users and for agents reading the source)

1. First sentence = the contract, self-contained. IDE quick-doc and LLM/agent context windows both key off the first line. Write "Returns the column at index as an AnyCol, throwing if out of bounds." — not "Gets a column." Never rely on "see above"; each rendered KDoc must stand alone.
2. Always document the failure mode with @throws. State the exact exception and the condition (IllegalArgumentException when the column/index is absent). For *OrNull accessors say explicitly that they return null instead. Agents pick the right overload from this alone.
3. Disambiguate overloads by how the argument selects. With ~7 getColumn overloads sharing a name, the useful information is name vs path vs index vs selector and the return type — lead with that so the reader (human or model) knows which one they want.
4. Cross-link with @see. Pair each throwing accessor with its *OrNull sibling and with the underlying get / getColumnOrNull. Good link graphs are what let an agent traverse the source instead of guessing.
5. One minimal example per family, not per overload. Use the canonical df receiver and a realistic column name; show the resolved type in a comment (// ColumnGroup<*>). Examples are the highest-signal part of a KDoc for both audiences.
6. Name the concrete return types with links ([ColumnGroup], [FrameColumn], [ColumnWithPath]), not just "a column" — resolvable types beat prose for tooling and for agents resolving symbols.
7. Call out the non-obvious edge cases the signature can't show: getColumnPath/getColumnWithPath require a single match; getOrNull tolerates negative indices; getColumns(vararg String) returns them in selector order.
8. Don't restate the signature. Add the semantics the types don't convey (interpretation, ordering, laziness, throwing). A KDoc that only re-types the signature is noise for both users and agents.

Acceptance criteria

• All non-deprecated accessors listed under Scope have concise KDocs; properties() is preserved.
• Each accessor documents its return type/interpretation and its throw-vs-null behavior (@throws or explicit null).
• Throwing accessors are @see-linked to their *OrNull counterparts where they exist.
• No documentation is added for deprecated / @AccessApiOverload overloads.