Add KDocs for `constructors.kt`

# Add KDocs for `constructors.kt`

Add missing KDocs for public APIs in `core/generated-sources/src/main/kotlin/org/jetbrains/kotlinx/dataframe/api/constructors.kt`.

Cover main constructor groups:

- `column`, `valueColumn`, `columnGroup`, `frameColumn`
- `columnOf`, `column`
- `dataFrameOf`, `DataFrameBuilder`, `DynamicDataFrameBuilder`
- `emptyDataFrame`, `pathOf`

Mention key behavior where relevant: type inference, generated/duplicate names, nested column groups/frame columns, and builder-style dataframe creation.

Use existing docs/examples from `createDataFrame.md` and tests in `constructors.kt` as references.

Start in core/generated-sources/src/main/kotlin/org/jetbrains/kotlinx/dataframe/api/constructors.kt and review the existing examples in createDataFrame.md plus the tests in constructors.kt. Add KDocs for the listed constructor groups, covering type inference, generated or duplicate names, nested column groups and frame columns, and builder-style dataframe creation. Done means the public APIs have complete, consistent documentation.

Written by the indexing model from the issue text.