docs: Add example of unpivoting multiple sets of columns (#18513)

[mdavis-xyz]

Solves #18513

[mdavis-xyz]

I haven't built the docs locally yet. I'm trying. It seems that building the docs requires compilation of the whole library itself. Is that right?

[mdavis-xyz]

Ok I've found out how to render the docs. But I can't see how to navigate to the API docs. All the links, search results etc link to the live prod website, not localhost.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.92%. Comparing base (6076421) to head (9ef0f14).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #18519      +/-   ##
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- Coverage   79.93%   79.92%   -0.02%     
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  Files        1505     1505              
  Lines      202628   202628              
  Branches     2873     2873              
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- Hits       161976   161942      -34     
- Misses      40104    40138      +34     
  Partials      548      548              

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[MarcoGorelli]

probably - for docs I normally cd to py-polars/docs and do make html then cd build/html and python -m http.server, then navigate to localhost:8000

could you rewrite the example without the extra square bracket, i.e.

        ...     .select(
        ...         "PRODUCT",
        ...         pl.struct(QTY="QUANTITY1", PRICE="PRICE1").alias("PQ1"),
        ...         pl.struct(QTY="QUANTITY2", PRICE="PRICE2").alias("PQ2")
        ...     )

and run make pre-commit too please?

then I think it should be good to go, this seems like a clear improvement

---

(and also make the same change for DataFrame.unpivot?)

[mdavis-xyz]

Done. And I also made the same changes to DataFrame.unpivot.