docs: add playground

Author: RexSkz

demo/index.less

@@ -38,6 +38,20 @@ a:visited {
     }
   }
 
+  .banner {
+    display: inline-block;
+    padding: 8px 16px;
+    border-radius: 4px;
+    background: #000;
+    color: #fff;
+    cursor: pointer;
+
+    &:hover {
+      background: #333;
+      text-decoration: underline;
+    }
+  }
+
   blockquote {
     margin: 0 0 1em;
     line-height: 24px;