Proposal is to create a separate repository to cover Artifact Distribution.
Accounts for: Artifact registry support #65

The new repository would explain:

• how distribution is used to store additional types.
• guidance to registry owners for how to parse artifact types
• guidance to artifact authors for how manifests and index are used to define new types

With artifacts defined here, the distribution spec can be updated to remove references to images, making it more about storing index and manifest references to layers.

Proposed repository name:
artifact-spec