Add X Activity API docs

Author: tcaldwell-x

docs.json

@@ -847,6 +847,18 @@
                   "x-api/account-activity/create-subscription"
                 ]
               },
+              {
+                "group": "X Activity",
+                "pages": [
+                  "x-api/activity/introduction",
+                  "x-api/activity/quickstart",
+                  "x-api/activity/activity-stream",
+                  "x-api/activity/create-x-activity-subscription",
+                  "x-api/activity/deletes-x-activity-subscription",
+                  "x-api/activity/get-x-activity-subscriptions",
+                  "x-api/activity/update-x-activity-subscription"
+                ]
+              },
               {
                 "group": "Filtered Stream Webhooks",
                   "pages": [