Updated marginalizing-models explain to use the model transformations

[zaxtax]

This updates the marginalizing models example notebook to use the new model transformation functionality in pymc-devs/pymc-extras#388

• Notebook follows style guide https://docs.pymc.io/en/latest/contributing/jupyter_style.html
• Check the notebook is not excluded from any pre-commit check: https://github.com/pymc-devs/pymc-examples/blob/main/.pre-commit-config.yaml

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📚 Documentation preview 📚: https://pymc-examples--764.org.readthedocs.build/en/764/

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[juanitorduz]

This looks great and should push it out. As a small suggestion: remove the warning and some verbose output (it might distract the reader)