Add redirects from old v3 notebooks

[OriolAbril]

This adds some redirects from the old notebooks.

Some of the notebooks (those core ones) are still in the pymc repo, so we can redirect
straight there. The sphinx redirect extension doesn't allow external redirects,
so they are redirected to
https://www.pymc.io/projects/docs/en/stable/learn/core_notebooks/index.html (which will now also
have a note about the wider example notebooks page).

closes #6706

Checklist

• Explain important implementation details 👆
• Make sure that the pre-commit linting/style checks pass.
• Link relevant issues (preferably in nice commit messages)
• Fill out the short summary sections 👇

Documentation

• Add redirects from old v3 notebooks.

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📚 Documentation preview 📚: https://pymc--6719.org.readthedocs.build/en/6719/