add pole placement example

[baggepinnen]

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[JuliaControlBot]

This is an automated message.
Plots were compared to references. 3/11 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
✔️ 0.0
✔️ 0.0
✔️ 0.0

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.59%. Comparing base (c522a79) to head (ec028b6).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1010   +/-   ##
=======================================
  Coverage   92.59%   92.59%           
=======================================
  Files          41       41           
  Lines        5116     5116           
=======================================
  Hits         4737     4737           
  Misses        379      379           

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