docs: add hm script

Author: tangjinzhou

site/index.html

@@ -18,11 +18,13 @@
     </style>
     <script>
         var _hmt = _hmt || [];
+        var isGithub = location.host.indexOf('github') !== -1;
+        var src = isGithub ? 'https://hm.baidu.com/hm.js?b3ef688fa86bfb75027f1b410180a867' : 'https://hm.baidu.com/hm.js?1564b57c5b8f74933e4fedca9dc75b0d';
         (function() {
-            const hm = document.createElement('script')
-            hm.src = 'https://hm.baidu.com/hm.js?b3ef688fa86bfb75027f1b410180a867'
-            const s = document.getElementsByTagName('script')[0]
-            s.parentNode.insertBefore(hm, s)
+            var hm = document.createElement('script');
+            hm.src = src;
+            var s = document.getElementsByTagName('script')[0];
+            s.parentNode.insertBefore(hm, s);
         })()
     </script>
 </head>
@@ -50,4 +52,4 @@
     </script>
 </body>
 
-</html>
+</html>