Add SAGE extension in doc

[blegat]

This illustrate how PolyJuMP can be extended using SAGE certificate as example. The possibility to do this was the reason we split SumOfSquares and PolyJuMP as separate packages.

[chriscoey]

cool

[codecov[bot]]

Codecov Report

Merging #240 (b6debdc) into master (37edbcb) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #240   +/-   ##
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  Coverage   76.70%   76.70%           
=======================================
  Files          36       36           
  Lines        1696     1696           
=======================================
  Hits         1301     1301           
  Misses        395      395           

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[blegat]

Superseeded by https://github.com/jump-dev/PolyJuMP.jl/pull/99