update json feed extensions with about field;

update json feed event extension to rename fields to not have `date_` prefix

json-schema/json-feed-extensions/rsvps/v1.json

@@ -4,6 +4,11 @@
   "type": "object",
   "description": "Contains additional Item metadata for Items that represent RSVPs to events",
   "properties": {
+    "about": {
+      "type": "string",
+      "format": "uri",
+      "description": "URI pointing to more information about this \"_rsvp\" extension"
+    },
     "rsvp": {
       "type": "string",
       "enum": [

json-schema/json-feed-extensions/rsvps/v1.yaml

@@ -3,6 +3,10 @@ $id: https://gitea.jbrumond.me/minimal/docs/raw/branch/master/json-schema/json-f
 type: object
 description: Contains additional Item metadata for Items that represent RSVPs to events
 properties:
+  about:
+    type: string
+    format: uri
+    description: URI pointing to more information about this "_rsvp" extension
   rsvp:
     type: string
     enum: