Add docs on numerical performance and accuracy

[baggepinnen]

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[JuliaControlBot]

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Plots were compared to references. 7/11 images have changed, see differences below.
After pulling this PR, please update the reference images by creating a PR to ControlExamplePlots.jl here.

Difference	Reference Image	New Image
❌ 0.048
✔️ 0.001
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✔️ 0.0
❌ 0.043
❌ 0.046
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[codecov]

Codecov Report

Merging #686 (beab56b) into master (0e73f34) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #686   +/-   ##
=======================================
  Coverage   87.45%   87.45%           
=======================================
  Files          34       34           
  Lines        3579     3579           
=======================================
  Hits         3130     3130           
  Misses        449      449           

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