docs: update extension examples

demos/src/Extensions/Collaboration/Vue/index.vue

@@ -8,6 +8,7 @@ import Document from '@tiptap/extension-document'
 import Paragraph from '@tiptap/extension-paragraph'
 import Text from '@tiptap/extension-text'
 import Collaboration from '@tiptap/extension-collaboration'
+import Placeholder from '@tiptap/extension-placeholder'
 import * as Y from 'yjs'
 import { WebrtcProvider } from 'y-webrtc'
 
@@ -35,6 +36,9 @@ export default {
         Collaboration.configure({
           document: ydoc,
         }),
+        Placeholder.configure({
+          placeholder: 'Write something … It’ll be shared with everyone else looking at this example.',
+        }),
       ],
     })
   },
@@ -45,3 +49,21 @@ export default {
   },
 }
 </script>
+
+<style lang="scss">
+/* Basic editor styles */
+.ProseMirror {
+  > * + * {
+    margin-top: 0.75em;
+  }
+}
+
+/* Placeholder (at the top) */
+.ProseMirror p.is-editor-empty:first-child::before {
+  content: attr(data-placeholder);
+  float: left;
+  color: #adb5bd;
+  pointer-events: none;
+  height: 0;
+}
+</style>