Add diff and docs to the 1.12.1 change log entry

(cherry picked from commit c9e803e)

Author: jgonggrijp

index.html

@@ -2713,7 +2713,7 @@ <h2 id="changelog">Change Log</h2>
       </p>
 
       <p id="1.12.1">
-        <b class="header">1.12.1</b> &mdash; <small><i>March 15, 2021</i></small><br />
+        <b class="header">1.12.1</b> &mdash; <small><i>March 15, 2021</i></small> &mdash; <a href="https://github.com/jashkenas/underscore/compare/1.12.0...1.12.1">Diff</a> &mdash; <a href="https://cdn.statically.io/gh/jashkenas/underscore/1.12.1/index.html">Docs</a><br />
         <ul>
           <li>
             Fixes a security issue in <tt>_.template</tt> that could enable a