add docs on creating delay system

[baggepinnen]

also improve the rest of the docs for creating systems slightly

[JuliaControlBot]

This is an automated message.
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 #681 (875a9d4) into master (a4523df) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #681   +/-   ##
=======================================
  Coverage   87.33%   87.33%           
=======================================
  Files          34       34           
  Lines        3521     3521           
=======================================
  Hits         3075     3075           
  Misses        446      446           

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