chore: add static v6 information to docVersions (#536)

Author: josephperrott

material.angular.io/src/app/shared/version/version.ts

@@ -15,6 +15,10 @@ export const docVersions: VersionInfo[] = [
     url: 'https://v5.material.angular.io/',
     title: '5.2.5'
   },
+  {
+    url: 'https://v6.material.angular.io/',
+    title: '6.4.7'
+  },
   {
     url: `https://material.angular.io/`,
     title: materialVersion