doc: zh_CN: index files in filesystems subdirectory

Add filesystems subdirectory into the table of Contents for zh_CN,
all translations residing on it would be indexed conveniently.

Reviewed-by: Alex Shi <alex.shi@linux.alibaba.com>
Reviewed-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Wang Wenhu <wenhu.wang@vivo.com>
Link: https://lore.kernel.org/r/20200316110143.97848-1-wenhu.wang@vivo.com


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