Update model averaging example

[aloctavodia]

This updates many elements

• First the model/dataset, partially because we got essentially one model dominating over the others and partially because we make a reference to Statistical Rethinking but on the second edition the example is used in a slightly different context.
• The text has many references to functions, method, defaults etc. No longer valid
• Concepts are better explained now, including a slightly longer discussion about when model averaging is a good idea and when not.

Closes #688

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📚 Documentation preview 📚: https://pymc-examples--696.org.readthedocs.build/en/696/

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