Reorganize and document in README file ostree and bootc blueprint structure. See USHIFT-4720: Implement bootc mirror registry cache for image builds for a bootc-related example of such work.

For ostree blueprints, it's important to keep in mind the parenting of the images now that we have an explicit `parent = image_name` directive (see USHIFT-4740: Add explicit parent directive option in ostree blueprints)

See USHIFT-4720: Implement bootc mirror registry cache for image builds as an example of bootc-related new structure.