Generate PDF versions of slides

[katherinehebert]

We've had a request to add an option to view and download the workshop slides as PDF, for people who want to annotate, take notes, or have a local copy to refer back to.

The newly released package renderthis would give us this option. We could add this to our rendering workflow, and add a PDF button to the website and readme pages for each workshop: https://www.jhelvy.com/posts/2022-06-28-introducing-renderthis/