doc: Move SLUB documentation to the admin guide

This section is supposed to be for internal documentation, while the
document is advice for sysadmins.  Move it to the appropriate place.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Acked-by: Harry Yoo <harry.yoo@oracle.com>
Link: https://patch.msgid.link/20250611155916.2579160-2-willy@infradead.org


Signed-off-by: Vlastimil Babka <vbabka@suse.cz>