bootc image requirements

bootc claims to be able to use an "OCI format" archive/image to deploy a system and lifecycle the thing through its paces. We all know that is glossing over some subtle but super important details concerning exactly how that image is structured and what content it has. This needs to be documented and as far as I can tell it really is not documented at all ([current docs](https://containers.github.io/bootc/building/guidance.html) do not do this subject justice).