Add documentation for Shakti C reference platform.

Signed-off-by: Vijai Kumar K <vijai@behindbytes.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20210412174248.8668-1-vijai@behindbytes.com
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
[ Changes from Bin Meng:
 - Add missing TOC
 Message-id: 20210430070534.1487242-1-bmeng.cn@gmail.com
]
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>