Add support for sidebar3.md, and do general cleanups

$cd .../docs/source.android.com/

$git checkout master
$python scripts/build.py
[...]
$find out/ -type f | sort | xargs md5sum > old_script.txt
$md5sum old_script.txt
cb96f8ae932fd7444409bd774eb5775a  old_script.txt

$git checkout cleanup
$python scripts/build.py
[...]
$find out/ -type f | sort | xargs md5sum > new_script.txt
$md5sum new_script.txt
cb96f8ae932fd7444409bd774eb5775a  new_script.txt

Change-Id: Ie85d2879df75b8f27a32f64a0768596ce4026ca0
diff --git a/templates/sidebar b/templates/sidebar
index 16e8000..748a2fc 100644
--- a/templates/sidebar
+++ b/templates/sidebar
@@ -1,4 +1,4 @@
 <div id=sidebar>
-  $sidebar$sidebar2
+  $sidebar$sidebar2$sidebar3
 </div>