doc: adaptive MovingHorizonEstimator on pH neutralization NonLinModelDAE tutorial

[franckgaga]

This PR add an adaptive MovingHorizonEstimator example on the pH neutralization process, which is a NonLinModelDAE plant model. The acid feed flow rate $q_{Ain}$ is measured, but not its concentration $c_{Ain}$. It is thus estimated in real-time with the MHE. It does not perfectly converge to the actual value, but it's expected since the excitation is quite low (persistency of excitation). Nonetheless, the estimation performances seem good enough for closed-loop control.

here's the preview : https://juliacontrol.github.io/ModelPredictiveControl.jl/previews/PR457

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.24%. Comparing base (2af3149) to head (f8e54a4).
⚠️ Report is 17 commits behind head on main.

Additional details and impacted files

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  Coverage   97.24%   97.24%           
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  Files          28       28           
  Lines        6867     6867           
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  Hits         6678     6678           
  Misses        189      189           

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unittests	97.24% <ø> (ø)

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