docs: cputopology: move the sysfs ABI description to right place

Documentation/admin-guide/cputopology.rst is the wrong place to describe
sysfs ABI. So move the cputopology ABI things to
Documentation/ABI/stable/sysfs-devices-system-cpu and add a reference to
ABI doc in Documentation/admin-guide/cputopology.rst.

Link: https://lkml.kernel.org/r/20210319041618.14316-1-song.bao.hua@hisilicon.com
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Tian Tao <tiantao6@hisilicon.com>
Signed-off-by: Barry Song <song.bao.hua@hisilicon.com>
Link: https://lore.kernel.org/r/20210611052249.25776-1-song.bao.hua@hisilicon.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>