Add KDocs for `append` and `appendNulls`

`core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/api/append.kt` has no KDocs for `DataFrame.append` and `DataFrame.appendNulls`.

The site page exists for `append`: https://kotlin.github.io/dataframe/append.html, but the public APIs should also document their behavior directly in KDoc.

Please add concise KDocs that explain:

- `append` returns a new `DataFrame` with one or more rows appended.
- `append` assigns values row-wise, and the number of values must be a multiple of the dataframe column count.
- `appendNulls` returns a new `DataFrame` with the requested number of rows filled with `null` values.
- `appendNulls(0)` returns the original dataframe.
- Both operations do not mutate the original dataframe.
- Since these operations create a new dataframe/columns, appending rows one by one in a loop is an antipattern; users should prefer constructing data first or appending/concatenating in batches.

### Acceptance criteria

- `DataFrame.append(vararg values: Any?)` has KDoc.
- `DataFrame.appendNulls(numberOfRows: Int = 1)` has KDoc.
- KDocs are consistent with the existing site documentation.
- KDocs explicitly mention immutability and the performance caveat for repeated appends in loops.