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>
diff --git a/CREDITS b/CREDITS
index beac0c8..401c509 100644
--- a/CREDITS
+++ b/CREDITS
@@ -3120,7 +3120,7 @@
 N: Rik van Riel
 E: riel@redhat.com
 W: http://www.surriel.com/
-D: Linux-MM site, Documentation/sysctl/*, swap/mm readaround
+D: Linux-MM site, Documentation/admin-guide/sysctl/*, swap/mm readaround
 D: kswapd fixes, random kernel hacker, rmap VM,
 D: nl.linux.org administrator, minor scheduler additions
 S: Red Hat Boston