docs/zh_CN: add mips to zh_CN index

All the files in the mips directory have been translated into
Chinese and it is time to add them to the zh_CN index.
Signed-off-by: Yanteng Si <siyanteng@loongson.cn>
Reviewed-by: Alex Shi <alexs@kernel.org>
Link: https://lore.kernel.org/r/20210406070239.19910-7-siyanteng@loongson.cnSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/translations/zh_CN/index.rst

@@ -23,6 +23,7 @@
    arm64/index
    sound/index
    cpu-freq/index
+   mips/index
 
 目录和表格
 ----------