Welcome, Warden!
You've found the little corner of the internet where we're trying to document the Prison Architect API.
There are two sections, really. Above, you can see a folder browser - in there we've dumped some of the game's main config files and images that are widely referred to when modding.
The bulk of the guide, however, is over in the wiki - you can get to it any time by clicking the little book icon in the sidebar, or just click here.