doc: zh_CN: index files in arm64 subdirectory

Add arm64 subdirectory into the table of Contents for zh_CN,
then add other translations in arm64 conveniently.

Signed-off-by: Bailu Lin <bailu.lin@vivo.com>
Reviewed-by: Alex Shi <alex.shi@linux.alibaba.com>
Link: https://lore.kernel.org/r/20200926022558.46232-1-bailu.lin@vivo.com


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