doc: board: anbernic: Update rgxx3 to add new boards

Update the RGxx3 documentation to note that it now supports the
RG-ARC-D, RG-ARC-S, Powkiddy RK2023, and Powkiddy RGB30. Also update
verbiage around panel detection to note that it is no longer hard coded
to the RG503.

Signed-off-by: Chris Morgan <macromorgan@hotmail.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>