docs/system/loongarch: Update the LoongArch document

docs/system/loongarch: Update the LoongArch document

1. Add some information about how to boot the LoongArch virt
machine by uefi bios and linux kernel and how to access the
source code or binary file.
2. Move the explanation of LoongArch system emulation in the
target/loongarch/README to docs/system/loongarch/loongson3.rst

Signed-off-by: Xiaojuan Yang <yangxiaojuan@loongson.cn>
Reviewed-by: Song Gao <gaosong@loongson.cn>
Message-Id: <20220812091957.3338126-1-yangxiaojuan@loongson.cn>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>