GitHub - theforeman/theforeman.org: The new and improved Foreman website. see the README there which explains how to build site and file PRs.
I am not sure I follow your comments above, but let me comment on this statement. You can indeed install CentOS 7.2 and then have it upgraded to 7.4. Some users prefer highly “locked” environments and don’t upgrade too much, only manually picked errata. So this is real life workflow for some, might not be good fit to you, sure that is fine. This is by the way what Red Hat customers pay for (EUS channels - ability to “stay” on let’s say RHEL 7.2 for a longer period of time and “skip” few releases to save money with ugprading).
Anyway, here is a wrap up. This is a kickstart tree (thus installation media): http://mirror.centos.org/centos/7.4.1708/os/x86_64/ (older versions like 7.3 are now moved to http://vault.centos.org but still available there). You basically mirror this once and set installation media.
Beware that CentOS mirrors publish a symlink here CentOS Mirror but what you see is really this: http://mirror.centos.org/centos/7.4.1708/os/x86_64/ and I do not recommend to use this URL for installation media as it is being changed after each release.
The repository you want to manage content for is CentOS Mirror and it is continious yum repo with all the updates across various CentOS releases.
The way it is implemented today is that Katello overrides IM and provides own URL there, OS is still needed and it is automatically created when you sync a “kickstart tree” repository. I think this works fine, proposals for improvement would be nice.