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

chrome/common/extensions/docs/templates/intros/webRequest.html

 <p id="classSummary">
 Use the <code>chrome.webRequest</code> module to intercept, block,
 or modify requests in-flight and to observe and analyze traffic.
 </p>
 
 <h2 id="manifest">Manifest</h2>
 <p>You must declare the "webRequest" permission in the <a
   href="manifest.html">extension manifest</a> to use the web request
-API, along with <a href="manifest.html#permissions">host permissions</a>
+API, along with <a href="declare_permissions.html">host permissions</a>
 for any hosts whose network requests you want to access. If you want to
 use the web request API in a blocking fashion, you need to request
 the "webRequestBlocking" permission in addition.
 For example:</p>
 <pre>{
   "name": "My extension",
   ...
   <b>"permissions": [
     "webRequest",
     "*://*.google.com"
@@ skipping 92 matching lines @@
   <li>Pragma</li>
   <li>Proxy-Authorization</li>
   <li>Proxy-Connection</li>
   <li>Transfer-Encoding</li>
 </ul>
 </p>
 
 <p>
 The webRequest API only exposes requests that the extension has
 permission to see, given its
-<a href="manifest.html#permissions">host permissions</a>.
+<a href="declare_permissions.html">host permissions</a>.
 Moreover, only the following schemes are accessible:
 <code>http://</code>,
 <code>https://</code>,
 <code>ftp://</code>,
 <code>file://</code>, or
 <code>chrome-extension://</code>.
 In addition, even certain requests with URLs using one of the above schemes
 are hidden, e.g.,
 <code>chrome-extension://other_extension_id</code> where
 <code>other_extension_id</code> is not the ID of the extension to handle
@@ skipping 157 matching lines @@
       }
     }
     return {requestHeaders: details.requestHeaders};
   },
   {urls: ["&lt;all_urls&gt;"]},
   ["blocking", "requestHeaders"]);
 </pre>
 
 <p> For more example code, see the <a href="samples.html#webrequest">web request
 samples</a>.</p>