Link the memory allocation guide from the MM docs

I just went looking for the memory allocation guide in the MM docs instead
of in the core API.  For the benefit of the next person who makes that
mistake, link to it from the MM docs.

Signed-off-by: Matthew Wilcox <willy@infradead.org>
Acked-by: Mike Rapoport <rppt@linux.ibm.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>