MAINTAINERS: add vm.rst to memory management core

MAINTAINERS: add vm.rst to memory management core

The vm.rst file is currently not listed in the MAINTAINERS file, so let's
go ahead and add to the MM core subsystem so that the maintainers are CCed
when changes to the documentation are proposed.

Link: https://lore.kernel.org/20260528-mm-vm-rst-maintainers-file-v1-1-306631c0a610@redhat.com
Signed-off-by: Brian Masney <bmasney@redhat.com>
Reviewed-by: Lorenzo Stoakes <ljs@kernel.org>
Reviewed-by: Oscar Salvador (SUSE) <osalvador@kernel.org>
Reviewed-by: Liam R. Howlett (Oracle) <liam@infradead.org>
Reviewed-by: SeongJae Park <sj@kernel.org>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>