fix: 增加 flutter 链接

aio/content/navigation.json

@@ -23,6 +23,10 @@
     {
       "url": "translations/cn/home",
       "title": "关于中文版"
+    },
+    {
+      "url": "https://flutter-io.cn/",
+      "title": "友站：Flutter CN"
     }
   ],
   "TopBarNarrow": [
@@ -666,6 +670,11 @@
       "tooltip": "Ant Design Mobile 的 Angular 实现，服务于无线产品。",
       "url": "https://ng.mobile.ant.design/"
     },
+    {
+      "url": "https://flutter-io.cn/",
+      "tooltip": "Flutter 中文文档站",
+      "title": "友站：Flutter 中文"
+    },
     {
       "url": "guide/change-log",
       "title": "Change Log",