doc: Update SelectorGroupChat doc on how to use O3-mini model.

[ekzhu]

Resolves #5408

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.56%. Comparing base (a226966) to head (65a2b38).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5657   +/-   ##
=======================================
  Coverage   75.56%   75.56%           
=======================================
  Files         171      171           
  Lines       10483    10483           
=======================================
  Hits         7921     7921           
  Misses       2562     2562           

Flag	Coverage Δ
unittests	75.56% <ø> (ø)

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