Add core notebook on sample_posterior_predictive for out-of-model predictions

[ricardoV94]

Description

We wrote a series of examples for the PyMC labs blogpost: https://www.pymc-labs.io/blog-posts/out-of-model-predictions-with-pymc/

It would be good to write a more technical companion that explains exactly how sample_posterior_predictive works and why it works this way, and when is it valid/not valid to use if for out-of-model predictions.

There is a long thread here that highlights aspects of the current behavior that are unclear / surprising for most users (including developers): #6047

Note: This is not a beginner friendly issue.