docs: cgroup-v1: use numbered lists for user interface setup

Setup instructions for memory resource controller UI uses a mix of
section headings and normal paragraphs, whereas numbered lists are
better fit for this purpose.

While at it, also slightly reword the instructions and add reference to
"Why are cgroups needed?" in the main cgroups documentation.

Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

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

Those files belong to the admin guide, so add them.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>