[Extensions Doc]Link Fixes

Quick change to two links in get started document:
/manifest linked to /apps/manifest - corrected to /extensions/manifest
Background page link sent to 404 - updated

TOC contained "packaging" which has a redirect to "Hosting and Updating"
Removed it from side nav.

Change-Id: I06a927a3d4cb52707aa5fdc214435482bedac5e2
Reviewed-on: https://chromium-review.googlesource.com/1045446Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: Crystal Lambert <crystallambert@chromium.org>
Cr-Commit-Position: refs/heads/master@{#556254}

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

@@ -30,7 +30,7 @@
 </p>
 <h2 id="manifest">Create the Manifest</h2>
 <p>
-  Extensions start with their <a href="/manifest">manifest</a>.
+  Extensions start with their <a href="extensions/manifest">manifest</a>.
   Create a file called <code>manifest.json</code>
   and include the following code,
   or download the file
@@ -85,7 +85,7 @@
 <p>
   Although the extension has been installed,
   it has no instruction.
-  Introduce a <a href="/background_page.html">background script</a>
+  Introduce a <a href="/background_pages.html">background script</a>
   by creating a file titled <code>background.js</code>,
   or downloading it
   <a href="examples/tutorials/get_started/background.js"

chrome/common/extensions/docs/templates/json/chrome_sidenav.json

@@ -183,10 +183,6 @@
                 "title": "Hosting Policy Changes",
                 "href": "/extensions/hosting_changes"
               },
-              {
-                "title": "Packaging",
-                "href": "/extensions/packaging"
-              },
               {
                 "title": "One-Time Payments",
                 "href": "/webstore/one_time_payments"