chore: demo preview fixed

src/sites/doc/components/demo-preview/demo-preview.scss

 .doc {
   &-demo-preview {
     height: 667px;
+    // height: 637px;
     width: 375px;
     position: absolute;
     right: 30px;
     top: 240px;
-    -webkit-box-shadow: #ebedf0 0 4px 12px;
+    // top: 135px;
     box-shadow: #ebedf0 0 4px 12px;
     border-radius: 12px;
     overflow: hidden;
+    &.fixed {
+      position: fixed;
+      top: 120px;
+    }
     iframe {
+      // height: calc(100% - 40px);
       height: 100%;
       width: 100%;
     }