Add meta tags for docs

docs/index.html

@@ -6,6 +6,10 @@
   <meta http-equiv="X-UA-Compatible" content="IE=edge,chrome=1" />
   <meta name="description" content="Description">
   <meta name="viewport" content="width=device-width, user-scalable=no, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0">  
+  <meta property="og:title" content="QMK Firmware Docs">
+  <meta property="og:description" content="The full documenation of the open-source firmware.">
+  <meta property="og:image" content="https://i.imgur.com/svjvIrw.jpg">
+  <meta property="og:url" content="https://docs.qmk.fm">
   <link rel="stylesheet" href="//unpkg.com/docsify/lib/themes/vue.css" title="light">
   <link rel="stylesheet" href="qmk.css" title="dark" disabled>
   <link rel="stylesheet" href="sidebar.css" />