Add polynomial optimization example

[blegat]

Preview https://jump-dev.github.io/SumOfSquares.jl/previews/PR271

[codecov]

Codecov Report

Base: 78.46% // Head: 76.77% // Decreases project coverage by -1.69% ⚠️

Coverage data is based on head (09ec3fe) compared to base (8cd8f96).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #271      +/-   ##
==========================================
- Coverage   78.46%   76.77%   -1.69%     
==========================================
  Files          36       36              
  Lines        1788     1602     -186     
==========================================
- Hits         1403     1230     -173     
+ Misses        385      372      -13     

Impacted Files	Coverage Δ
src/Bridges/Constraint/psd2x2.jl	37.50% <0.00%> (-4.81%)	⬇️
src/constraint.jl	70.40% <0.00%> (-4.81%)	⬇️
src/sosdec.jl	92.64% <0.00%> (-4.73%)	⬇️
src/Certificate/Sparsity/ideal.jl	81.81% <0.00%> (-3.90%)	⬇️
src/attributes.jl	68.75% <0.00%> (-3.48%)	⬇️
src/Certificate/Sparsity/xor_space.jl	92.30% <0.00%> (-3.15%)	⬇️
src/Bridges/Variable/copositive_inner.jl	63.15% <0.00%> (-2.42%)	⬇️
src/Certificate/Certificate.jl	96.82% <0.00%> (-1.90%)	⬇️
src/Bridges/Variable/scaled_diagonally_dominant.jl	68.42% <0.00%> (-1.35%)	⬇️
src/Bridges/Constraint/diagonally_dominant.jl	90.42% <0.00%> (-1.17%)	⬇️
... and 20 more

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