Fix some typos, broken links and other issues in extension docs

chrome/common/extensions/docs/templates/articles/background_pages.html

 <h1>Background Pages</h1>
 
 
 <p id="eventPageWarning" class="warning">
   <em>Caution:</em> Consider using event pages instead.
   <a href="event_pages.html">Learn more</a>.
 </p>
 
 <p>
 A common need for extensions is to have
@@ skipping 61 matching lines @@
   <b>"background": {
     "page": "background.html"
   }</b>,
   ...
 }</pre>
 
 <p>
 If you need the browser to start up early&mdash;so
 you can display notifications, for example&mdash;then
 you might also want to specify the
-<a href="manifest.html#permissions">"background" permission</a>.
+<a href="declare_permissions.html#bg">"background" permission</a>.
 </p>
 
 
 <h2 id="details">Details</h2>
 
 <p>
 You can communicate between your various pages using direct script calls,
 similar to how frames can communicate.
 The $ref:extension.getViews method
 returns a list of window objects
@@ skipping 58 matching lines @@
   &lt;body>
     &lt;p>
     Image here:
     &lt;/p>
 
     &lt;img id="target" src="white.png" width="640" height="480">
 
   &lt;/body>
 &lt;/html>
 </pre>