Improve README.md and docu

[ufechner7]

• fix link
• add page Development
• add diagram

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 61.59%. Comparing base (7f35215) to head (4c2a16e).
Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #48   +/-   ##
=======================================
  Coverage   61.59%   61.59%           
=======================================
  Files          15       15           
  Lines         677      677           
=======================================
  Hits          417      417           
  Misses        260      260           

Flag	Coverage Δ
unittests	61.59% <ø> (ø)

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[ufechner7]

@davidanthoff Please, review!