it's a GitHub Pages site in the @GitHub org from a repo called gh-stack (i.e. the repo is at https://github.com/github/gh-stack/).

There’s a special case where certain official orgs can continue to use github.com instead of github.io for their Pages domain, and that’s how you end up with:

https://github.github.com/gh-stack/

from the code:

Should Pages owned by this user be regarded as “Official GitHub properties”?

def github_owned_pages? GitHub.github_owned_pages.include?(login) end

# Orgs/users that are owned by GitHub and should be allowed to use # `github.com` URLs. # # Returns an Array of String User/Organization logins. ...