Update master for stable/xena

Add file to the reno documentation build to show release notes for
stable/xena.

Use pbr instruction to increment the minor version number
automatically so that master versions are higher than the versions on
stable/xena.

Sem-Ver: feature
Change-Id: Ie1e7be2d3c88b4ba376d4d6a2f4c8ee01a752dcd

releasenotes/source/index.rst

@@ -6,6 +6,7 @@
     :maxdepth: 1
 
     unreleased
+    xena
     wallaby
     victoria
     ussuri

releasenotes/source/xena.rst

+=========================
+Xena Series Release Notes
+=========================
+
+.. release-notes::
+   :branch: stable/xena