Translated the RISC-V architecture boot documentation.

The patch adds a new file boot.rst to the Documentation/translations/zh_CN/
arch/riscv/ directory, and adds a reference to the new file
in the index.rst file.

Signed-off-by: longjin <longjin@DragonOS.org>
Reviewed-by: Yanteng Si <siyanteng@loongson.cn>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20231218092924.200165-1-longjin@DragonOS.org