Update blackbox likelihood example

[ricardoV94]

Changes

• Make Op more generic (accepts distinct inputs, returns vectorized loglikelihood), for better transfer learning
• Build Ops more iteratively, and show useful debugging strategies
• Use CustomDist instead of Potential (added a short section at the end showing Potential)
• Actually make use of finite differences as mentioned in the text
• Remove mention of old Cython example and reference new JAX example
• Fix some cross-references (unclear success)

Checklist

• [x] Notebook follows style guide https://docs.pymc.io/en/latest/contributing/jupyter_style.html
• [ ] PR description contains a link to the relevant issue:
  • a tracker one for existing notebooks (tracker issues have the "tracker id" label)
  • or a proposal one for new notebooks
• [x] Check the notebook is not excluded from any pre-commit check: https://github.com/pymc-devs/pymc-examples/blob/main/.pre-commit-config.yaml

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📚 Documentation preview 📚: https://pymc-examples--631.org.readthedocs.build/en/631/