Editor open graph metadata (google#4527)

* Add href to AR button, improve accessibility

* Add Open Graph title and description for editor

* Broaden copy scope to include 3D, not just AR

packages/space-opera/editor/index.html

@@ -20,6 +20,8 @@
   <meta charset="utf-8">
   <meta name="description" content="&lt;model-viewer&gt; editor">
   <meta name="viewport" content="width=device-width, initial-scale=1">
+  <meta property="og:title" content='﹤model-viewer﹥ Editor - Edit your asset files for 3D & AR' />
+  <meta property="og:description" content="A tool to edit glTF and glb files ✏️, and configure them to use the ﹤model-viewer﹥ 3D & AR web component 🌐" />
   <title>Model Editor</title>
   <link href="https://fonts.googleapis.com/css?family=Material+Icons&display=block" rel="stylesheet">
   <link rel="shortcut icon" type="image/png" href="../../shared-assets/icons/favicon.png"/>