[DOC] Tutorial: variable-length columns (ListArray and BatchArray)

12.batcharray.ipynb covers BatchArray basics but there is no tutorial for ListArray — the newer, more flexible varlen column type. Users working with nested or ragged data (e.g. GPS waypoints, tags, embeddings of varying length) have no worked example to follow.

Suggested work: Extend 12.batcharray.ipynb or add 12b.listarray.ipynb covering:
- Creating a ListArray with item_spec
- Appending, extending, and fancy-indexing
- Embedding a ListArray as a column inside a CTable (using the dataclass schema API)
- Arrow round-trip (from_arrow / to_arrow)
- When to prefer ListArray over BatchArray