[project page] Add some links to top paragraph

Author: marijnh

doc/manual.html

@@ -2045,7 +2045,7 @@ <h3 id="api_static">Static properties</h3>
 </section>
 
 <section id=addons>
-    <h2>Addons</h2>
+    <h2 id="addons">Addons</h2>
 
     <p>The <code>addon</code> directory in the distribution contains a
     number of reusable components that implement extra editor

index.html

@@ -40,7 +40,7 @@
 <section id=description class=first>
   <p><strong>CodeMirror</strong> is a versatile text editor
   implemented in JavaScript for the browser. It is specialized for
-  editing code, and comes with a number of language modes and addons
+  editing code, and comes with a number of <a href="mode/index.html">language modes</a> and <a href="doc/manual.html#addons">addons</a>
   that implement more advanced editing functionality.</p>
 
   <p>A rich <a href="doc/manual.html#api">programming API</a> and a