docs/vm: rename documentation files to .rst

Documentation/ABI/stable/sysfs-devices-node

Documentation/ABI/testing/sysfs-kernel-mm-hugepages

Documentation/ABI/testing/sysfs-kernel-mm-ksm

Documentation/ABI/testing/sysfs-kernel-slab

Documentation/admin-guide/kernel-parameters.txt

Documentation/dev-tools/kasan.rst

Documentation/filesystems/proc.txt

Documentation/filesystems/tmpfs.txt

Documentation/sysctl/vm.txt

Documentation/vm/00-INDEX

Documentation/vm/active_mm.rst

Documentation/vm/balance.rst

Documentation/vm/cleancache.rst

Documentation/vm/frontswap.rst

Documentation/vm/highmem.rst

Documentation/vm/hmm.rst

Documentation/vm/hugetlbfs_reserv.rst

Documentation/vm/hugetlbpage.rst

Documentation/vm/hwpoison.rst

Documentation/vm/idle_page_tracking.rst