Improve website docs for `constructors.kt` APIs

# Improve website docs for `constructors.kt` APIs

Improve website coverage for public APIs from
`core/generated-sources/src/main/kotlin/org/jetbrains/kotlinx/dataframe/api/constructors.kt`.

Update existing pages where appropriate:

- `docs/StardustDocs/topics/createDataFrame.md`
- `docs/StardustDocs/topics/createColumn.md`

Cover DataFrame creation APIs on `Create DataFrame`:

- `emptyDataFrame`
- `dataFrameOf`
- `DataFrameBuilder`
- `DynamicDataFrameBuilder`

Cover DataColumn creation APIs on `Create DataColumn`:

- `columnOf`
- `column(values: Iterable)`

Also decide where to document column reference/path constructors:

- `column`
- `valueColumn`
- `columnGroup`
- `frameColumn`
- `ColumnDelegate`
- `pathOf`

Recommended improvements:

- Clarify which APIs create actual data (`DataFrame`, `DataColumn`) and which create references/accessors.
- Add builder-style examples for `dataFrameOf(...).fill`, `fillIndexed`, `withColumns`, and random generator methods.
- Add examples for value columns, column groups, and frame columns.
- Mention generated names and duplicate-name behavior where relevant.
- Cross-link `Create DataFrame`, `Create DataColumn`, and column access/reference documentation where useful.
- Keep examples consistent with constructor tests and current behavior.