New spec.md links to other documents

Prior to 1.0, lets make sure all the new .md documents are linked to from spec.md

Please see https://github.com/opencontainers/distribution-spec/pull/178#discussion_r479472819 & https://github.com/opencontainers/distribution-spec/pull/178#discussion_r479499242 & https://github.com/opencontainers/distribution-spec/pull/178#discussion_r479501246

