Move release notes into releasenotes and add tox target so that
infra scripts will publish them to
docs.openstack.org/releasenotes/openstack-manuals
Infra change: Icbe135e37c78e0a3a1c6d3243f7128c0b881325d
Note that the directory layout here is this way on purpose - we want to
reuse the same jobs other repos use, just the difference is we're not
using reno for this.
Change-Id: I00a53d8b3d9bef28b480be22af2e6647fa26448a