It was custom made for the tutorial. I didn't release it yet because I mostly focused on polishing the tutorial itself and not the code for the interface. Do you think people would be interested in the interface by itself?

I was wondering the same, this format feels extremely intuitive for the material it explores. Looking at the HTML and the CSS class names I'd assume it was handmade, but it seems like standardizing this format or even generating this webpage format from a repo would be a useful tool. The comment left at the end is ambiguous as to whether it refers to the tutorial or the website as a whole.