Add KDocs (and assess tests/site coverage) for `typeConversions` APIs

[zaleslaw]

Add KDocs (and assess tests/site coverage) for typeConversions APIs

The file core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/api/typeConversions.kt is a large collection of conversion / cast helpers (toColumn*, asColumnGroup, asFrameColumn, asNumbers, castToNullable, toValueColumn, asGroupBy, toMap, toDataRow, array conversions, etc.). It is partially documented. Please add concise KDocs for the remaining non-deprecated APIs, excluding deprecated / @AccessApiOverload overloads.

Already documented (leave as-is, or lightly align)

• SingleColumn<C>.asColumnGroup() (both overloads) — shared KDoc anchor SingleColumnAsColumnGroupDocs.
• The Infer enum and its entries (None, Nulls, Type).
• The NullabilityOptions enum and its entries (Infer, Checking, Widening).
• NullabilityOptions.applyNullability(...).

Not documented — scope (grouped)

• Accessor builders — String / ColumnPath / ColumnReference / KProperty → toColumnAccessor, toColumnOf, toColumnGroupAccessor, toFrameColumnAccessor.
• Type views / casts on columns — asNumbers, asComparable (on DataColumn, ColumnSet, SingleColumn), castToNullable, castToNotNullable, setNullable.
• Column-kind conversions — asValueColumn, asColumnGroup, asFrameColumn, asDataColumn, asDataFrame, FrameColumn.toValueColumn, BaseColumn.toDataFrame.
• To arrays — toTypedArray, toFloatArray / toDoubleArray / toIntArray / toLongArray / toShortArray / toByteArray.
• Iterable / Array → columns/paths — toValueColumn, toColumn, toColumnOf, toFrameColumn, toColumnGroup / toColumnGroupOf, toPath.
• DataFrame / DataRow / Map conversions — DataFrame.toMap, DataFrame.asColumnGroup, DataFrame.asGroupBy (all overloads), DataRow.toDataFrame, DataRow.toMap, Map.toDataRow.
• The NullabilityException class.

KDocs should explain

• What each helper converts from → to, and whether it is a cheap cast/view (no data copied, may throw / be unchecked) or an actual conversion/copy — e.g. asColumnGroup / asNumbers just cast, while toValueColumn / toColumn build a new column, and toXxxArray copy values.
• Requirements / failure modes where relevant (e.g. asNumbers requires isNumber(), castToNotNullable throws if the column has nulls).
• The distinction inside overload families that differ only by receiver (String vs ColumnPath vs KProperty).
• The infer: Infer / name parameters on the toColumn / toValueColumn builders.

Test coverage assessment

• There is no dedicated test file for typeConversions.kt (no core/src/test/.../api/typeConversions.kt).
• These helpers are exercised only indirectly / incidentally across many tests (samples like Create.kt / Modify.kt / Analyze.kt, the testSets/person suites, toDataFrame.kt, etc.).
• Recommendation: treat this ticket as KDocs-first, but note that adding a focused test file covering the non-trivial helpers (asNumbers/asComparable requirements, castToNotNullable on a column with nulls, toXxxArray, asGroupBy, toMap/toDataRow round-trip) would be a worthwhile follow-up.

Website coverage assessment

• There is no dedicated site page for these conversions. They appear only fragmentarily on operation pages: asGroupBy on groupBy.html, various toColumn / creation helpers on createColumn.html, and mentions in convert.html, ColumnSelectors.html, replace.html.
• Recommendation: KDocs are the primary deliverable here. A small "type conversions / casts" section (or cross-links from createColumn.html) could be a follow-up, but is out of scope unless requested.

Acceptance criteria

• All non-deprecated APIs in typeConversions.kt listed under "Not documented" have concise KDocs; the already-documented items are preserved.
• No documentation is added for deprecated / @AccessApiOverload overloads.
• KDocs make the cast/view vs conversion/copy distinction explicit and state failure modes (require/throws) where they exist.
• The ticket's test-coverage and website findings above are recorded (a follow-up test file / site section may be filed separately).