Artifact Registry ADR-007: document repository-delete cascade behavior

Documents the repository-delete cascade behavior in Artifact Registry ADR-007: the per-format child and collection-link FKs to repositories use ON DELETE CASCADE, while artifact-to-child FKs stay NO ACTION so a non-empty repository cannot be deleted. Implements the decision behind gitlab-org/ops/artifact-registry!702 (merged). Also pads three pre-existing MD060 table-delimiter rows in the same file (handbook CI lints the whole modified file); no content change.