docs: Expand streaming user guide with 6 real-world patterns

[ThotDjehuty]

Summary

The streaming page currently only covers collect(engine="streaming") and show_graph(). This PR expands it with actionable examples for the full streaming API surface.

What's added

streaming.py — 6 new tagged code sections:

Tag	Pattern
larger_than_ram	Realistic group_by/agg showing the chunk-based memory model
sink_parquet	ETL write: scan CSV → enrich → sink_parquet (full output never in RAM)
sink_csv	Aggregation → sink_csv
sink_batches	Per-batch callback: count rows in each chunk
collect_async	Two concurrent queries via asyncio.gather + collect_async()
partition_pruning	Multi-file scan_parquet with predicate pushdown skipping non-matching files

streaming.md — expanded from ~1,500 to ~4,000 bytes:

• API summary table at the top (all 6 streaming APIs in one view)
• ASCII diagram of the chunk-based memory pipeline
• "Writing in streaming mode: sink_*" section (sink_parquet + sink_csv)
• "Per-batch callbacks: sink_batches" section
• "Concurrent async execution: collect_async" section
• "Multi-file scans and partition pruning" section with Hive-partition best-practices note

The hidden comment <!-- Not included in the docs "until we have something we are proud of" --> has been removed now that the page has substantive content.

Testing

All examples are self-contained (iris fixture + tempfile.TemporaryDirectory), requiring no external network access. Compatible with the existing docs CI.

Related

#20947 (streaming engine tracking issue)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.42%. Comparing base (0a50a14) to head (696fbdb).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

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- Coverage   81.43%   81.42%   -0.02%     
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  Files        1801     1801              
  Lines      246750   246750              
  Branches     3081     3081              
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- Hits       200936   200908      -28     
- Misses      45028    45056      +28     
  Partials      786      786              

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