boards: amlogic: update documentation for Beelink GT-King/Pro

Update the device matrix and add build instructions.

Signed-off-by: Christian Hewitt <christianshewitt@gmail.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
diff --git a/doc/board/amlogic/index.rst b/doc/board/amlogic/index.rst
index a585d85..275c366 100644
--- a/doc/board/amlogic/index.rst
+++ b/doc/board/amlogic/index.rst
@@ -17,7 +17,7 @@
 +===============================+===========+=================+==============+============+============+=============+==============+
 | Boards                       	| Odroid-C2 | P212            | Khadas VIM2  | S400       | U200       | Odroid-N2   | SEI610       |
 |        			| Nanopi-K2 | Khadas-VIM      | Libretech-PC |	          | SEI510     | Khadas-VIM3 | Khadas-VIM3L |
-|        			| P200	    | LibreTech-CC v1 |	WeTek Core2  |	          |            |	     | Odroid-C4    |
+|        			| P200	    | LibreTech-CC v1 |	WeTek Core2  |	          |            | GT-King/Pro | Odroid-C4    |
 |        			| P201	    | LibreTech-AC v2 |	             |	          |            |             |              |
 +-------------------------------+-----------+-----------------+--------------+------------+------------+-------------+--------------+
 | UART                       	| **Yes**   | **Yes**         | **Yes**      | **Yes**    | **Yes**    | **Yes**     | **Yes**      |
@@ -77,6 +77,8 @@
 .. toctree::
    :maxdepth: 1
 
+   beelink-gtking
+   beelink-gtkingpro
    khadas-vim2
    khadas-vim3l
    khadas-vim3