Update master for stable/zed

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

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

Sem-Ver: feature
Change-Id: I8f24a2318b5bd5ff60a235c093db022344dec644

releasenotes/source/index.rst

@@ -8,6 +8,7 @@ Contents
    :maxdepth: 2
 
    unreleased
+   zed
    yoga
    xena
    wallaby

releasenotes/source/zed.rst

+========================
+Zed Series Release Notes
+========================
+
+.. release-notes::
+   :branch: stable/zed