Documentation: enable ditaa integration

For prettier diagrams: http://ditaa.sourceforge.net/

Change-Id: Ic28dc5ea9d82ff6bf8654e2e33e675a536348654
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/37646
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

util/docker/doc.coreboot.org/ditaa.sh

@@ -0,0 +1,2 @@
+#!/bin/sh
+exec java -jar /usr/lib/ditaa0_9.jar $*