doc: boards: amlogic: add documentation for Beelink GT1 Ultimate

Add build instructions for the Beelink GT1 Ultimate board.

Signed-off-by: Christian Hewitt <christianshewitt@gmail.com>
Signed-off-by: Karl Chan <exxxxkc@getgoogleoff.me>
Link: https://lore.kernel.org/r/8760391a-3506-5ac5-8d55-92c23cccff44@yahoo.com
[narmstrong: fixed MAINTAINERS indentation, added doc to index]
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
diff --git a/doc/board/amlogic/index.rst b/doc/board/amlogic/index.rst
index deb7976..66b581c 100644
--- a/doc/board/amlogic/index.rst
+++ b/doc/board/amlogic/index.rst
@@ -90,6 +90,7 @@
    bananapi-m2s
    bananapi-m5
    beelink-gskingx
+   beelink-gt1-ultimate
    beelink-gtking
    beelink-gtkingpro
    jethub-j80