Update AR ntoebook to PyMC 5

[fonnesbeck]

{Insert Description}

Updated AR notebook to v5 and also changed the example to have an underlying AR(2) process, which makes it a little more interesting.

• [ ] 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
• [ ] Check the notebook is not excluded from any pre-commit check: https://github.com/pymc-devs/pymc-examples/blob/main/.pre-commit-config.yaml

Helpful links

• https://github.com/pymc-devs/pymc-examples/blob/main/CONTRIBUTING.md

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[fonnesbeck]

Bad rebase. Closing.