docs: Added documation for on_columns for LazyFrames

[nicholaslegrand102]

Fixes #26831

First-time contributor here!
This PR adds documentation to the pivot section to inform users that they can specify on_columns when pivoting a LazyFrame, allowing them to perform the pivot without calling collect() beforehand.

Ai Use: I used Claude to help me simplify my documentation modifications.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.63%. Comparing base (8f2501b) to head (1f9d878).
⚠️ Report is 20 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #26859      +/-   ##
==========================================
+ Coverage   81.62%   81.63%   +0.01%     
==========================================
  Files        1806     1806              
  Lines      248038   248038              
  Branches     3122     3122              
==========================================
+ Hits       202462   202495      +33     
+ Misses      44770    44737      -33     
  Partials      806      806              

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