Documentation/zh_CN: Add basic LoongArch documentations

Add some basic documentation (zh_CN version) for LoongArch. LoongArch is
a new RISC ISA, which is a bit like MIPS or RISC-V. LoongArch includes a
reduced 32-bit version (LA32R), a standard 32-bit version (LA32S) and a
64-bit version (LA64).

Reviewed-by: Alex Shi <alexs@kernel.org>
Reviewed-by: Yanteng Si <siyanteng@loongson.cn>
Reviewed-by: Guo Ren <guoren@kernel.org>
Reviewed-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Co-developed-by: WANG Xuerui <git@xen0n.name>
Signed-off-by: WANG Xuerui <git@xen0n.name>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>