Reorganized left-nav (part of Apps doc revamp effort).

STAGED DOCS: https://chrome-apps-doc.appspot.com/_patch/23190012/apps/about_apps.html

R=kalman@chromium.org, mangini@chromium.org

Review URL: https://codereview.chromium.org/23190012

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@229725 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 229725