docs: zh_CN: create the architecture-specific top-level directory

This mirrors commit 4f1bb038 ("docs: create a top-level arch/
directory"), creating a top-level directory to hold architecture-specific
documentation.

Acked-by: Alex Shi <alexs@kernel.org>
Reviewed-by: Yanteng Si <siyanteng@loongson.cn>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>