doc/zh_CN: add mips index.rst translation

This patch translates Documentation/mips/index.rst into Chinese.
Signed-off-by: Yanteng Si <siyanteng@loongson.cn>
Reviewed-by: Alex Shi <alex.shi@linux.alibaba.com>
Reviewed-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Link: https://lore.kernel.org/r/20210112115259.217944-1-siyanteng@loongson.cnSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/translations/zh_CN/mips/index.rst

+.. SPDX-License-Identifier: GPL-2.0
+
+.. include:: ../disclaimer-zh_CN.rst
+
+:Original: :ref:`Documentation/mips/index.rst <index>`
+:Translator: Yanteng Si <siyanteng@loongson.cn>
+
+.. _cn_index:
+
+
+===========================
+MIPS特性文档
+===========================
+
+.. toctree::
+   :maxdepth: 2
+   :numbered:
+
+   booting
+   ingenic-tcu
+
+   features
+
+.. only::  subproject and html
+
+   Indices
+   =======
+
+   * :ref:`genindex`