docs: cgroup-v1: add it to the admin-guide book

Documentation/admin-guide/cgroup-v1/blkio-controller.rst

Documentation/admin-guide/cgroup-v1/cgroups.rst

Documentation/admin-guide/cgroup-v1/cpuacct.rst

Documentation/admin-guide/cgroup-v1/cpusets.rst

Documentation/admin-guide/cgroup-v1/devices.rst

Documentation/admin-guide/cgroup-v1/freezer-subsystem.rst

Documentation/admin-guide/cgroup-v1/hugetlb.rst

Documentation/admin-guide/cgroup-v1/index.rst

Documentation/admin-guide/cgroup-v1/memcg_test.rst

Documentation/admin-guide/cgroup-v1/memory.rst

Documentation/admin-guide/cgroup-v1/net_cls.rst

Documentation/admin-guide/cgroup-v1/net_prio.rst

Documentation/admin-guide/cgroup-v1/pids.rst

Documentation/admin-guide/cgroup-v1/rdma.rst

Documentation/admin-guide/cgroup-v2.rst

Documentation/admin-guide/index.rst

Documentation/admin-guide/kernel-parameters.txt

Documentation/admin-guide/mm/numa_memory_policy.rst

Documentation/block/bfq-iosched.rst

Documentation/filesystems/tmpfs.txt