Add QCQP to polynomial optimization example

[blegat]

Preview https://jump-dev.github.io/SumOfSquares.jl/previews/PR318

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.10%. Comparing base (246c7ca) to head (9b01262).
⚠️ Report is 44 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #318   +/-   ##
=======================================
  Coverage   85.10%   85.10%           
=======================================
  Files          41       41           
  Lines        2222     2222           
=======================================
  Hits         1891     1891           
  Misses        331      331           

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