docs/zh_CN: add highmem translation

Translate .../vm/highmem.rst into Chenese.

Signed-off-by: Yanteng Si <siyanteng@loongson.cn>
Reviewed-by: Alex Shi <alexs@kernel.org>
Link: https://lore.kernel.org/r/51e088d1e8659b9411534a5a3ad03d88c79a5297.1643246827.git.siyanteng@loongson.cn


Signed-off-by: Jonathan Corbet <corbet@lwn.net>