update schema

Author: ricardo-devis-agullo

website/static/schema.json

@@ -2,6 +2,13 @@
   "$schema": "http://json-schema.org/draft-07/schema#",
   "type": "object",
   "properties": {
+    "$schema": {
+      "description": "A field for the [JSON schema](https://json-schema.org/) specification",
+      "type": [
+        "string",
+        "null"
+      ]
+    },
     "registries": {
       "type": "array",
       "items": {