Looking at the page source, it doesn't appear to be Wordpress; none of the usual cruft that WP scatters through the source (though it could have been processed via a filter).
Looking at the request/response headers in Firefox's web developer tools, it's gone through Cloudflare, but there's also a x-github header, so maybe some custom tooling that's pushed content to github?
Looking at the request/response headers in Firefox's web developer tools, it's gone through Cloudflare, but there's also a x-github header, so maybe some custom tooling that's pushed content to github?