I recommend clicking that big, exciting "fork" button in the upper-right. Then you'll own the code.
You'll notice we're on a special gh-pages branch, meaning that the contents of this repository are available at http://appstop.github.io/<this-repo-name>. It may take a while for Github to make the page available. After you fork it, it'll be available at http://<your-username>.github.io/<this-repo-name>.
Once you've forked this repository into your own account, you can modify the HTML or set up and a custom domain using the instructions here.