Fix What's New TOC to only animate the sliding movement on hide

Previously it was animating on font size change which is distracting and
inconsistent with the actual content.

Author: ychin

release-notes/whatsnew.html

@@ -120,7 +120,7 @@
             line-height: 150%;
             font-size:110%;
 
-            transition: 0.3s;
+            transition: right 0.3s;
         }
         .toc ul {
             list-style-type: none;