Add a README file to the "mathematical operations" examples folder

[luisaFelixSalles]

This PR allows this example section to be shown in the documentation

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.62%. Comparing base (17bc61f) to head (b8090b1).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1874      +/-   ##
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+ Coverage   87.51%   88.62%   +1.10%     
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  Files          90       90              
  Lines       10220    10220              
==========================================
+ Hits         8944     9057     +113     
+ Misses       1276     1163     -113     

[PProfizi]

Hey @luisaFelixSalles, the example ends-up mixed with basic examples

I think you need to renumber the folder for mathematical examples or something like that.

[PProfizi]

See comment about example ending-up with basic examples

[PProfizi]

Doc renders correctly