Generate a container layout spec

It'd be good to try to create a more formal spec for what we expect in OCI images and where.

Right now we have this: https://bootc.dev/bootc/bootc-images.html

It could be cleaned up a bit. We do need to extend this w/composefs sealed UKIs in `/boot` for sure.

When defining a spec we should be clear about consumers. IMO a good one for this is https://github.com/bootc-dev/bcvk/

We could also start to stub out what a "flat" layout container image looks like - this relates to e.g. https://bootc.dev/bootc/experimental-container-export.html

Finally, related to the above there's also some discussion about a direct container -> ISO approach without involving `bootc install`.