Move registry api definitions under registry package

Signed-off-by: Stephen J Day <stephen.day@docker.com>

docs/api/v2/doc.go

@@ -0,0 +1,9 @@
+// Package v2 describes routes, urls and the error codes used in the Docker
+// Registry JSON HTTP API V2. In addition to declarations, descriptors are
+// provided for routes and error codes that can be used for implementation and
+// automatically generating documentation.
+//
+// Definitions here are considered to be locked down for the V2 registry api.
+// Any changes must be considered carefully and should not proceed without a
+// change proposal in docker core.
+package v2