Update PyMC tutorial notebooks with improvements and fixes

[atheendre130505]

Description

This PR updates the PyMC tutorial notebooks with various improvements and fixes to enhance clarity, correctness, and user experience.

Changes

• Fixed outdated code snippets for compatibility with latest PyMC version
• Improved explanations and added clarifications in markdown cells
• Enhanced notebook structure for better readability
• Ensured all notebook cells execute without errors

Testing

• All updated notebooks were executed end-to-end without failures
• Verified that outputs and plots render correctly

Notes

• If maintainers prefer, notebook updates can be split into smaller PRs or moved to the pymc-examples repository.

Related Issue

No specific issue linked.

Checklist

• [x] Follows PyMC contributing guidelines
• [x] Executed all cells without exceptions
• [x] Documentation improvements only, no API changes

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📚 Documentation preview 📚: https://pymc--7952.org.readthedocs.build/en/7952/