cgroup: replace unified-hierarchy.txt with a proper cgroup v2 documentation

Now that cgroup v2 is almost out of the door, replace the development
documentation unified-hierarchy.txt with Documentation/cgroup.txt
which is a superset of unified-hierarchy.txt and authoritatively
describes all userland-visible aspects of cgroup.

v2: Updated to include all information from blkio-controller.txt and
    list filesystems which support cgroup writeback as suggested by
    Vivek.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>
Cc: Vivek Goyal <vgoyal@redhat.com>