Docs: Wrap VueAnnouncer in the ClientOnly component

Author: ktquez

docs/.vuepress/components/GlobalLayout.vue

@@ -1,6 +1,8 @@
 <template>
   <div id="global-layout">
-    <VueAnnouncer />
+    <ClientOnly>
+      <VueAnnouncer />
+    </ClientOnly>
     <component :is="layout"/>
   </div>
 </template>

docs/demos/vuepress.md

@@ -17,7 +17,9 @@ Ok, now just insert the component `<VueAnnouncer />` in your main layout.
 ```vue
 <template>
   <div>
-    <VueAnnouncer /> <!-- You can place it anywhere in your application -->
+    <ClientOnly>
+      <VueAnnouncer /> <!-- You can place it anywhere in your application -->
+    </ClientOnly>
     ...
   </div>
 </template>