Add screenshots to networking.config documentation.

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

 <h2 id="usage">Implementing a Captive Portal Authenticator</h2>
 This API allows an Extension to implement an authenticator for captive portals.
 The interaction between Extension, Chrome, and the user will be:
 <ol>
   <li>Using $(ref:setNetworkFilter), the Extension registers for a list of Wi-Fi
   networks that it wants to handle captive portals on.</li>
   <li>If one of these networks is detected by Chrome, it will appear in the
   network list, badged with an icon to indicate that a captive portal handler is
-  available.</li>
+  available.
+  <p>
+  <img src="{{static}}/images/networking_config_badge.png"
+     align="middle" width="301" height="305" alt="Badge icon in tray network list" />
+  </li>
   <li>If the user selects such a network, but a captive portal is not detected,
   Chrome will connect as usual - the Extension will never be involved.</li>
   <li>Otherwise, if a captive portal is detected, the user is notified and asked
   to authenticate using the registered Extension or whether to visit the captive
-  portal page.</li>
+  portal page.
+  <p>
+  <img src="{{static}}/images/networking_config_notification.png"
+     align="middle" width="367" height="224" alt="Captive portal notification" />
+  </li>
   <li>If the user selects the Extension, Chrome notifies the Extension through
   the $(ref:onCaptivePortalDetected) event.</li>
   <li>The Extension should now interact with the user if necessary and
   authenticate at the captive portal.</li>
   <li>Once the Extension finished the authentication, it notifies the API using
   $(ref:finishAuthentication) about its success or failure.</li>
 </ol>
 
 <p>
 In the Extension, this can look similar to the following snippet:
@@ skipping 41 matching lines @@
 function continueAuthentication(guid, credentials) {
   if (!credentials) {
     chrome.networking.config.finishAuthentication(guid, 'failed');
   }
   AuthenticateToMyCaptivePortalServer(credentials, function(success) {
     chrome.networking.config.finishAuthentication(
         guid, success ? 'succeeded' : 'rejected');
   });
 }
 </pre>