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

Documentation/arm64/index.rst

+.. _arm64_index:
+
 ==================
 ARM64 Architecture
 ==================

Documentation/translations/zh_CN/arm64/index.rst

+.. include:: ../disclaimer-zh_CN.rst
+
+:Original: :ref:`Documentation/arm64/index.rst <arm64_index>`
+:Translator: Bailu Lin <bailu.lin@vivo.com>
+
+.. _cn_arm64_index:
+
+
+==========
+ARM64 架构
+==========
+
+.. toctree::
+    :maxdepth: 2

Documentation/translations/zh_CN/index.rst

@@ -19,6 +19,7 @@
    admin-guide/index
    process/index
    filesystems/index
+   arm64/index
 
 目录和表格
 ----------