Improve docs and tests for `concat`

## Improve docs and tests for `concat`

`core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/api/concat.kt` contains many public `concat` APIs, but their KDocs, unit tests, and site documentation are incomplete.

Current site page: https://kotlin.github.io/dataframe/concat.html
Current test file: `core/src/test/kotlin/org/jetbrains/kotlinx/dataframe/api/concat.kt`

### Scope

- Add or improve KDocs for public `concat` overloads in `concat.kt`.
- Fix misleading KDoc in `GroupBy.concat()` if needed: it says keys are not included, but the return description mentions additional key columns.
- Improve unit tests for the main `concat` overloads and edge cases.
- Improve the site page with clearer examples and input/output tables.
- Align the site docs, KDocs, and tests around the same expected behavior.

### Areas to cover

- `DataFrame.concat(vararg frames)`
- `DataFrame concat frame`
- `DataFrame.concat(rows: Iterable>)`
- `DataFrame.concat(frames: Iterable>)`
- `DataColumn.concat(vararg other)`
- `DataColumn>.concat()`
- `DataColumn>.concat()`
- `DataRow.concat(vararg rows)`
- `GroupBy.concat()`
- `GroupBy.concatWithKeys()`
- `ReducedGroupBy.concat()`
- `Iterable>.concat()`
- `Iterable>.concat()`
- `Iterable?>.concat()`

### Acceptance criteria

- Public `concat` APIs have concise and accurate KDocs.
- Unit tests cover the main overload groups and documented behavior.
- Schema unification is tested and documented, including missing columns filled with `null`.
- `GroupBy.concat()` and `concatWithKeys()` behavior is clearly distinguished.
- Site page contains examples and input/output tables similar to other API pages.