Improve site documentation for `concat`

## Improve site documentation for `concat`

The current `concat` page is too brief and does not fully explain the main API variants:
https://kotlin.github.io/dataframe/concat.html

Improve the page to match the quality of other API documentation pages and clearly demonstrate the expected behavior.

### My recommendations

- Add input/output data tables for the main `DataFrame.concat` example.
- Add a clear schema unification example, including missing columns filled with `null`.
- Add examples for `DataColumn.concat`, `Iterable.concat()`, and `FrameColumn.concat()`.
- Explain the difference between `groupBy.concat()` and `groupBy.concatWithKeys()`.
- Make examples visually consistent with other API pages, with runnable code snippets and resulting tables.

### Acceptance criteria

- The page contains practical examples with input/output tables.
- Schema unification behavior is easy to understand from the examples.
- Main overload groups from `concat.kt` are represented.
- `groupBy.concat()` and `concatWithKeys()` are clearly distinguished.