doc: added C code generation example in "Manual: Linear Design"

[franckgaga]

This PR add a simple example of C code generation on the linear case study, the CSTR with feedforward compensation.

I tested the compilation locally on windows 11 and gcc. It works as expected and the printed u value is identical to the Julia version.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.57%. Comparing base (192cf70) to head (bf34a14).
⚠️ Report is 7 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #372   +/-   ##
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  Coverage   98.57%   98.57%           
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  Files          27       27           
  Lines        5468     5468           
=======================================
  Hits         5390     5390           
  Misses         78       78           

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