docs: admin-guide: move sysctl directory to it

The stuff under sysctl describes /sys interface from userspace
point of view. So, add it to the admin-guide and remove the
:orphan: from its index file.

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

Documentation/admin-guide/mm/index.rst

@@ -11,7 +11,7 @@ processes address space and many other cool things.
 Linux memory management is a complex system with many configurable
 settings. Most of these settings are available via ``/proc``
 filesystem and can be quired and adjusted using ``sysctl``. These APIs
-are described in Documentation/sysctl/vm.rst and in `man 5 proc`_.
+are described in Documentation/admin-guide/sysctl/vm.rst and in `man 5 proc`_.
 
 .. _man 5 proc: http://man7.org/linux/man-pages/man5/proc.5.html