docs: disable API playground for self-hosted compatibility (#295)

Sencho is self-hosted so the Mintlify "Try it" playground can never
reach a user's instance (CORS + localhost). Switch to "simple" mode
which keeps the endpoint URL/method as a copyable reference without
the misleading Send button.

docs/docs.json

@@ -71,6 +71,9 @@
     "auth": {
       "method": "bearer",
       "name": "Authorization"
+    },
+    "playground": {
+      "display": "simple"
     }
   },
   "navigation": {