Docs/admin-guide/mm/numaperf: increase depth of subsections

Each section of numaperf.rst has zero depth, and therefore be exposed to
the index of admin-guide/mm.  Especially 'See Also' section on the index
makes the document weird.  Hide the sections from the index by giving the
document a title and increasing the depth of each section.

[sj@kernel.org: change title to fix duplicate label warning]
  Link: https://lkml.kernel.org/r/20230106194927.152663-1-sj@kernel.org
Link: https://lkml.kernel.org/r/20230103180754.129637-6-sj@kernel.org


Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>