add gif demo

docs/assets/example/backlight.js

@@ -1,6 +1,7 @@
+var url = 'https://zhw2590582.github.io/assets-cdn';
 var art = new Artplayer({
     container: '.artplayer-app',
-    url: 'https://zhw2590582.github.io/assets-cdn/video/ambilight.mp4',
+    url: url + '/video/one-more-time-one-more-chance-480p.mp4',
     autoSize: true,
     moreVideoAttr: {
         crossOrigin: 'anonymous',