docs: document using papermill for notebook reproducibility

[Panaetius]

In the old docs we had a section detailing how to use papermill for reproducibility: https://renku.readthedocs.io/en/0.6.0/tutorials/firststeps.html#add-a-second-workflow-step

In the new docs flights tutorial, this section has been removed. As a replacement, we should add a Howto Guide to our docs detailing papermill usage.