Adding benchmark section in the docs, with a first simple example

[ludoro]

No description provided.

[codecov]

Codecov Report

Merging #199 into master will increase coverage by 0.18%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #199      +/-   ##
==========================================
+ Coverage   84.60%   84.78%   +0.18%     
==========================================
  Files          19       19              
  Lines        1604     1604              
==========================================
+ Hits         1357     1360       +3     
+ Misses        247      244       -3     

Impacted Files	Coverage Δ
src/Optimization.jl	90.45% <0.00%> (+0.53%)	⬆️

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