Update of Documentation/cgroups/00-INDEX

unified-hierarchy.txt was added by 65731578 (cgroup: add documentation
about unified hierarchy)

Cc: Tejun Heo <tj@kernel.org>
Cc: Li Zefan <lizefan@huawei.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: cgroups@vger.kernel.org
Cc: linux-doc@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Henrik Austad <henrik@austad.us>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>