I started with the demo jupyter-book and started to delete pages I didn't want.

I creaetd a new project repository and set GitHub pages to look for the source in the master branch. This means that the url is cranmer.github.io/intro-exp-phys-book. That didn't work at first, that URL was redirected to the introduction.html landing page automatically. Later I realized I needed to modify the _config.yml file to be:

baseurl: "/intro-exp-phys-book" # the subpath of your site, e.g. /blog. If there is no subpath for your site, use an empty string ""
url: "https://cranmer.github.io" # the base hostname & protocol for your site, e.g. http://example.com

Now it's working.