rockchip: ringneck_px30: migrate README to doc/board in rST format

This migrates the plaintext README in
board/theobroma-systems/ringneck_px30 to doc/board/theobroma-systems and
while doing so, update the instructions and rewrite it in rST.

Cc: Quentin Schulz <foss+uboot@0leil.net>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>