Add chrome.extension to extension_api.json and docs

chrome/common/extensions/docs/template/api_template.html

   <div id="container">
   <a name="top"> </a>
 
     <!-- API HEADER -->
     <div id="pageHeader">
       <!-- BREADCRUMB -->
       <!-- TODO: Fix these hrefs -->
       <div id="breadcrumbs">
         <a href="index.html">Google Chrome Extensions</a> &gt;
         <a href="reference_index.html">Reference</a> &gt;
@@ skipping 27 matching lines @@
           <li> <a href="index.html">Home</a></li>
           <li> <a href="getstarted.html">Get Started</a></li>
           <li> <a href="overview.html">Overview</a></li>
           <li>Reference 
             <ul>
               <li><a href="manifest.html">Manifest</a>
               <li><a href="toolstrip.html">Toolstrips</a>
               <li><a href="pageActions.html">Page actions</a>
               <li><a href="background_pages.html">Background pages</a>
               <li><a href="content_scripts.html">Content scripts</a>
+              <li><a href="extension.html">Extension</a>
               <li><a href="tabs.html">Tabs</a>
               <li><a href="windows.html">Windows</a>
               <li><a href="self.html">Self</a>
               <li><a href="npapi.html">NPAPI Plugins</a>
               <li><a href="packaging.html">Packaging</a>
               <li><a href="autoupdate.html">Autoupdate</a>
             </ul>
           <li>Samples
             <ul>
               <li>[sample 1]
@@ skipping 270 matching lines @@
           </div> <!-- /apiGroup -->
         </div> <!-- /apiPage -->
       </div> <!-- /mainColumn -->
     </div> <!-- /pageContent -->
     <div id="pageFooter" -->
       Copyright 2009
       <br />
       TBD: copyright/license should be automatically included here
     </div> <!-- /pageFooter -->
   </div> <!-- /container -->