Add Apache Arrow documentation page

[luisquintanilla]

Adds a docs page covering the Apache Arrow result streaming shipped in Giorgi/DuckDB.NET#334.

The page covers:

• ExecuteArrowStream returning an IArrowArrayStream, with batch/column access
• ExecuteArrowBatchesAsync with await foreach and a CancellationToken
• How the Arrow path honors UseStreamingMode for bounded memory
• The first-result-set limitation: Arrow methods return only the first result set, so statements after it do not run. Use ExecuteReader + NextResult for multiple result sets.

It adds two runnable samples (code/ArrowResultStream.cs, code/ArrowResultBatchesAsync.cs) and registers the page in docs/toc.yml after Basic Usage.

[luisquintanilla]

@Giorgi this one's ready for review. Thanks!