DOC: wire up Rosetta Stone tutorial into Learn → Core Notebooks

[hackforceone]

Title

Add Rosetta Stone tutorial comparing PyMC and Stan

Description

This PR adds a new tutorial notebook, Rosetta Stone, to the core documentation under Learn → Core Notebooks. The notebook shows side-by-side translations of equivalent model definitions in Stan and in PyMC, illustrating the minimal syntactic and conceptual differences between the two frameworks. It was originally authored as a gist (linked below) and has been reformatted and wired into the Sphinx/MyST-NB docs so it builds and renders alongside the other core notebooks.

Original gist:
https://gist.github.com/ricardoV94/c421ddacb3ba7a19ac46efa253ea466c

Related Issue

• Closes #
• Related to #

Checklist

• Checked that the pre-commit linting/style checks pass
• Included tests that prove the fix is effective or that the new feature works
• Added necessary documentation (example notebook in learn/core_notebooks/rosetta_stone_pymc_stan.ipynb)
• Each commit corresponds to a relevant logical change

Type of change

• Documentation
• New feature / enhancement
• Bug fix
• Maintenance
• Other (please specify)

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📚 Documentation preview 📚: https://pymc--7784.org.readthedocs.build/en/7784/

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