jsguide: add deprecation notice

Make it clear that the JS guide is no longer updated as Google is
migrating to TypeScript everywhere.

jsguide.html

@@ -11,6 +11,12 @@
 <body onload="initStyleGuide();">
 <div id="content">
 <h1>Google JavaScript Style Guide</h1>
+
+<p style="position: fixed; left: 0; top: 0; padding: 0.5em; margin: 0; background-color: #fef6e0; text-align: center; width: 100%; font-weight: bold; z-index: 1;">
+Please note: This guide is no longer being updated. Google recommends migrating
+to TypeScript, and following the <a href="tsguide.html">TypeScript guide</a>.
+</p>
+
 <h2 id="introduction">1 Introduction</h2>
 
 <p>This document serves as the <strong>complete</strong> definition of Google’s coding standards