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 <[e-mail]>
Reviewed-by: Alex Shi <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jonathan Corbet <[e-mail]>

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 <[e-mail]>
+
+.. _cn_arm64_index:
+
+
+==========
+ARM64 架构
+==========
+
+.. toctree::
+    :maxdepth: 2

Documentation/translations/zh_CN/index.rst

···
    admin-guide/index
    process/index
    filesystems/index
+   arm64/index
 
 目录和表格
 ----------