Moved registry docs to registry subdirectory

docs/spec/auth/index.md

@@ -0,0 +1,17 @@
+<!--[metadata]>
++++
+title = "Docker Registry Token Authentication"
+description = "Docker Registry v2 authentication schema"
+keywords = ["registry, on-prem, images, tags, repository, distribution, authentication, advanced"]
+[menu.main]
+parent="smn_registry_ref"
+weight=100
++++
+<![end-metadata]-->
+
+# Docker Registry v2 authentication
+
+See the [Token Authentication Specification](token.md),
+[Token Authentication Implementation](jwt.md),
+[Token Scope Documentation](scope.md),
+[OAuth2 Token Authentication](oauth.md) for more information.