feat: Update DataFrame.pivot to allow index=None when values is set

[MarcoGorelli]

closes #11592, though using a different solution than suggested - here I'm implementing "if index is not specified, use all remaining columns", just as is already done for values, as well as for unpivot. Related discussion #17095

I've also rewritten the pivot docstring examples, as I think they started off too complicated. It now starts with the simplest case, and then builds up

[codecov]

Codecov Report

Attention: Patch coverage is 97.56098% with 1 line in your changes missing coverage. Please review.

Project coverage is 80.87%. Comparing base (7f5412a) to head (23481be).
Report is 4 commits behind head on main.

❗ Current head 23481be differs from pull request most recent head eb89ca6

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Files	Patch %	Lines
crates/polars-lazy/src/frame/pivot.rs	50.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #17126   +/-   ##
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  Coverage   80.86%   80.87%           
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  Files        1456     1456           
  Lines      191337   191351   +14     
  Branches     2739     2740    +1     
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+ Hits       154721   154748   +27     
+ Misses      36109    36096   -13     
  Partials      507      507           

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[stinodego]

Looks great, thanks! Indeed, makes sense that either index or values can be None.

Good to merge when CI is green.