mb/intel/dg43gt: Add documentation

Change-Id: I4e9dc67e66f719d440679b11332e2c8a764024f4
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/28258
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Felix Held <felix-coreboot@felixheld.de>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

Documentation/mainboard/index.md

@@ -20,6 +20,10 @@ The boards in this section are not real mainboards, but emulators.
 
 - [Spike RISC-V emulator](emulation/spike-riscv.md)
 
+## Intel
+
+- [DG43GT](intel/dg43gt.md)
+
 ## Foxconn
 
 - [D41S](foxconn/d41s.md)