docs: Add ontology-driven design brief and architecture diagram

[JessUWE]

• design brief explaining ontology-driven architecture
• Architecture diagram showing class relationships and components

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.75%. Comparing base (d0cacce) to head (52ae5f5).

Additional details and impacted files

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##             main     #397   +/-   ##
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  Coverage   99.75%   99.75%           
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  Files          10       10           
  Lines        1602     1602           
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  Hits         1598     1598           
  Misses          4        4           

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