doc: MovingHorizonEstimator data widows in "Internals -> State Estimators"

[franckgaga]

To help future developments of the MHE. This is similar to the preamble in the "Public -> Functions -> Predictive Controllers" page. I put it in the internals section since it is more relevant to the developers than the users.

Here's a preview of the online documentation:

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.40%. Comparing base (b1a3fbc) to head (a221d3e).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

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##             main     #415   +/-   ##
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  Coverage   98.40%   98.40%           
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  Files          29       29           
  Lines        6130     6130           
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  Hits         6032     6032           
  Misses         98       98           

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