docs(@vant/use): add useRelation doc

packages/vant-use/.vuepress/config.js

@@ -33,6 +33,11 @@ module.exports = {
           '/src/useWindowSize/',
         ],
       },
+      {
+        title: 'Enhanced',
+        collapsable: false,
+        children: ['/src/useRelation/'],
+      },
     ],
   },
 };