Build the book on each PR

We need to find a way to take a look at the current state of the examples on each PR, to be able to review without having to build the notebooks locally. The first thing to try is to use the [gh-pages-deploy-action](https://github.com/JamesIves/github-pages-deploy-action) and deploy PR-specific versions to a `pr-{pr_number}` folder. We can remove this folder upon merging.