Cleaning up source.android.com files.

This change fixes some formatting artifacts that resulted from the export from
Sites, and also updates links to no longer point to Sites and use the standard
{@docRoot} idiom. Also contains a few content updates, and introduces a page
about branch management.

pdk/docs/downloads/index.jd

@@ -2,7 +2,6 @@ page.title=Downloads
 doc.type=downloads
 doc.hidenav=true
 @jd:body
-<div id="mainBodyFixed">
 <p>This page provides access to various downloads. Note that if you're looking
 for the Android SDK (for application developers), you should visit <a
 href="http://developer.android.com/sdk/index.html">developer.android.com</a>.</p>
@@ -43,4 +42,3 @@ there is no compatibility program for Android 1.5 and earlier. Note also that
 there is no compatibility program for Android 2.0, since it was superceded by
 Android 2.1 after only a few weeks.
 </p>
-</div>