docs: admin-guide: add cgroup-v2 documentation

The cgroup-v2.txt is already in ReST format. So, move it to the
admin-guide, where it belongs.

Cc: Li Zefan <lizefan@huawei.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/00-INDEX

@@ -80,8 +80,6 @@ cdrom/
 	- directory with information on the CD-ROM drivers that Linux has.
 cgroup-v1/
 	- cgroups v1 features, including cpusets and memory controller.
-cgroup-v2.txt
-	- cgroups v2 features, including cpusets and memory controller.
 cma/
 	- Continuous Memory Area (CMA) debugfs interface.
 conf.py

Documentation/admin-guide/index.rst

@@ -48,6 +48,7 @@ configure specific aspects of kernel behavior to your liking.
    :maxdepth: 1
 
    initrd
+   cgroup-v2
    serial-console
    braille-console
    parport