automate running example notebooks with github actions

We have a set of example notebooks that can be treated as end-to-end tests on real data and could possibly be automated using github actions.

Rough plan would be:
- update notebooks to output their results, which we then can compare with what we know is correct
- figure out how to automate running notebooks with github actions (e.g. [this one](https://github.com/marketplace/actions/run-notebook))