it's using github pages for the main website i think so i'm not sure if it's possible to setup custom domain + custom ssl cert for it?
Given that Jenkins is pretty popular, you'd think that they'd be able to sort something out with GitHub to get bumped up the list for something along these lines.
There's always the Cloudfare option, but I've never felt that this was an ideal solution when HTTPS should be extremely straightforward for GitHub to set up on their pages.