docs: (marimo) prettify onnx notebook

[superlopuh]

I would encourage cloning the branch and running the code locally to see the effect.
I think I'll migrate most of this notebook to one that doesn't depend on onnx, so we can have wasm links do it directly.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.85%. Comparing base (75b78ec) to head (4b3df02).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2880      +/-   ##
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- Coverage   89.85%   89.85%   -0.01%     
==========================================
  Files         399      399              
  Lines       50171    50172       +1     
  Branches     7755     7755              
==========================================
- Hits        45083    45082       -1     
- Misses       3860     3861       +1     
- Partials     1228     1229       +1     

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