doc: document iqb internals

[bassosimone]

This pull request documents the iqb internals. Most of the documentation consists of Jupyter Notebooks explaining usage and design choices. We also add tests that statically typecheck the notebooks. The intent of adding those tests is making sure that the they are still in sync with the codebase. In turn, that makes them living documentation.

The notebooks that appear in here are the same ones I used in an internal presentation on 2026-01-14. During this presentation, I explained to my colleagues the current design of the iqb codebase.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.18%. Comparing base (32940a5) to head (d6f4482).

Additional details and impacted files

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##             main     #130   +/-   ##
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  Coverage   99.17%   99.18%           
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  Files          37       38    +1     
  Lines        2296     2318   +22     
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+ Hits         2277     2299   +22     
  Misses         19       19           

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