docs/zh_CN: add riscv index.rst translation

This patch translates Documentation/riscv/index.rst into Chinese.
Signed-off-by: Yanteng Si <siyanteng@loongson.cn>
Reviewed-by: Alex Shi <alex.shi@linux.alibaba.com>
Link: https://lore.kernel.org/r/20210228191054.6048-5-siyanteng@loongson.cnSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/translations/zh_CN/riscv/index.rst

+.. SPDX-License-Identifier: GPL-2.0
+
+.. include:: ../disclaimer-zh_CN.rst
+
+:Original: :doc:`../../../riscv/index`
+:Translator: Yanteng Si <siyanteng@loongson.cn>
+
+.. _cn_riscv_index:
+
+
+===============
+RISC-V 体系结构
+===============
+
+.. toctree::
+    :maxdepth: 1
+
+    boot-image-header
+    pmu
+    patch-acceptance
+
+
+.. only::  subproject and html
+
+   目录
+   ====
+
+   * :ref:`genindex`