add Savage-Dickey Density Ratio example

[aloctavodia]

This needs an unreleased version of arviz

• 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 or a proposal one for new notebooks
• 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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[OriolAbril]

left one nit change after skimming the preview. I think I'l be able to accept the suggestion here on github and merge direcly though.