doc: adding a NMPC controller to the pH neutralization tutorial

[franckgaga]

The goal is to keep the pH of the effluent of the following CSTR neutralized:

The adaptive NonLinMPC controller works quite well.

Here's the preview: https://juliacontrol.github.io/ModelPredictiveControl.jl/previews/PR471

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.17%. Comparing base (c98c3ac) to head (b48c890).
⚠️ Report is 7 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #471   +/-   ##
=======================================
  Coverage   97.17%   97.17%           
=======================================
  Files          28       28           
  Lines        6985     6985           
=======================================
  Hits         6788     6788           
  Misses        197      197           

Flag	Coverage Δ
unittests	97.17% <ø> (ø)

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