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: I880c4ee202c2f70960cadc13b7e688dff03fdf30

releasenotes/source/index.rst

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

releasenotes/source/xena.rst

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