standardize release note page ordering

In order to support automatically updating the release notes when we
create stable branches, we want the pages to be in a standard order.
This patch updates the order to be reverse chronological, so the most
recent notes appear at the top.

Change-Id: I8d7bc165dd1c623498397ac9259f767b3d371172
diff --git a/releasenotes/source/index.rst b/releasenotes/source/index.rst
index 2c22408..0ec0e94 100644
--- a/releasenotes/source/index.rst
+++ b/releasenotes/source/index.rst
@@ -5,10 +5,10 @@
  .. toctree::
     :maxdepth: 1
 
+    unreleased
     v12.0.0
     v11.0.0
     v10.0.0
-    unreleased
 
 Indices and tables
 ==================