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

Documentation/filesystems/index.rst

+.. _filesystems_index:
+
 ===============================
 Filesystems in the Linux kernel
 ===============================

Documentation/translations/zh_CN/filesystems/index.rst

+.. SPDX-License-Identifier: GPL-2.0
+
+.. include:: ../disclaimer-zh_CN.rst
+
+:Original: :ref:`Documentation/filesystems/index.rst <filesystems_index>`
+:Translator: Wang Wenhu <wenhu.wang@vivo.com>
+
+.. _cn_filesystems_index:
+
+========================
+Linux Kernel中的文件系统
+========================
+
+这份正在开发的手册或许在未来某个辉煌的日子里以易懂的形式将Linux虚拟\
+文件系统（VFS）层以及基于其上的各种文件系统如何工作呈现给大家。当前\
+可以看到下面的内容。
+
+文件系统
+========
+
+文件系统实现文档。
+
+.. toctree::
+   :maxdepth: 2
+

Documentation/translations/zh_CN/index.rst

@@ -14,6 +14,7 @@
    :maxdepth: 2
 
    process/index
+   filesystems/index
 
 目录和表格
 ----------