docs: announce documentation migration

docs.json

@@ -806,6 +806,11 @@
             "label": "GitHub",
             "href": "https://github.com/langbot-app/LangBot"
           }
+        },
+        "banner": {
+          "content": "This documentation has moved to [langbot.app/docs](https://langbot.app/docs/en). This legacy site is no longer updated.",
+          "type": "warning",
+          "dismissible": false
         }
       },
       {
@@ -1117,6 +1122,11 @@
             "label": "GitHub",
             "href": "https://github.com/langbot-app/LangBot"
           }
+        },
+        "banner": {
+          "content": "本文档已迁移至 [langbot.app/docs](https://langbot.app/docs/zh)，此旧站不再更新。",
+          "type": "warning",
+          "dismissible": false
         }
       },
       {
@@ -1403,6 +1413,11 @@
             "label": "GitHub",
             "href": "https://github.com/langbot-app/LangBot"
           }
+        },
+        "banner": {
+          "content": "ドキュメントは [langbot.app/docs](https://langbot.app/docs/ja) に移行しました。この旧サイトは今後更新されません。",
+          "type": "warning",
+          "dismissible": false
         }
       }
     ]
@@ -1412,5 +1427,10 @@
   },
   "metadata": {
     "google-site-verification": "LBNmWbXCAXdoqrbwg5Gep-xzQ7u7lJHSdpR49N3Ttgg"
+  },
+  "banner": {
+    "content": "This documentation has moved to [langbot.app/docs](https://langbot.app/docs/en). This legacy site is no longer updated.",
+    "type": "warning",
+    "dismissible": false
   }
 }