update to commit 481cc973 ("mm,doc: Add new
documentation structure")

Note: There are some empty files that do not need to be updated for now,
they are:

bootmem,
oom,
page_allocation,
page_cache,
page_reclaim,
page_tables,
physical_memory
process_addrs,
shmfs,
slab,
vmalloc,
swap.
Signed-off-by: Yanteng Si <siyanteng@loongson.cn>
Reviewed-by: Wu XiangCheng <bobwxc@email.cn>
Reviewed-by: Alex Shi <alexs@kernel.org>
Link: https://lore.kernel.org/r/35d10864fb9b57bf288d4efa5642f93a60079029.1655362610.git.siyanteng@loongson.cnSigned-off-by: Jonathan Corbet <corbet@lwn.net>