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: I465c586eecf3dea6ad59a641f78b81a7a8da60cf

releasenotes/source/index.rst

@@ -6,6 +6,7 @@
    :maxdepth: 1
 
    unreleased
+   zed
    yoga
    xena
    wallaby

releasenotes/source/zed.rst

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