If I understand, the page at Hosting your website on a server (article) | Khan Academy means that I can use GitHub to host a forum for free if I don’t have my own computer, and I want to use a library computer. Is that right? Thank you.
In short: No. You can host a static website using GitHub Pages. But to run a forum you need to run code on the server, for example to process incoming posts and store them somewhere, maybe in database, which you’d also have to run.