Adds notebook to pandas-split-apply example

So that people can see it in the notebook e2e.

Changes

[ ] PR has an informative and human-readable title (this will be pulled into the release notes)
[ ] Changes are limited to a single goal (no scope creep)
[ ] Code passed the pre-commit check & code is left cleaner/nicer than when first encountered.
[ ] Any change in functionality is tested
[ ] New functions are documented (with a description, list of inputs, and expected output)
[ ] Placeholder code is flagged / future TODOs are captured in comments
[ ] Project documentation has been updated if adding/changing functionality.