add docs for nonlinear functionality

[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
✔️ 0.001
✔️ 0.0
❌ 0.043
❌ 0.046
✔️ 0.01

[codecov]

Codecov Report

Merging #671 (68a7db4) into master (2824aca) will not change coverage.
The diff coverage is 60.00%.

@@           Coverage Diff           @@
##           master     #671   +/-   ##
=======================================
  Coverage   87.33%   87.33%           
=======================================
  Files          34       34           
  Lines        3522     3522           
=======================================
  Hits         3076     3076           
  Misses        446      446           

Impacted Files	Coverage Δ
src/analysis.jl	88.44% <50.00%> (ø)
src/hammerstein_weiner.jl	92.12% <100.00%> (ø)

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