Documentation: Move ACPI documentation in a subindex

Change-Id: I17c5263674b805a73d98aaa3e7090083905e37ef
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/37242
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>

Documentation/acpi/index.md

@@ -0,0 +1,11 @@
+# ACPI-specific documentation
+
+This section contains documentation about coreboot on ACPI.
+
+## GPIO
+
+- [GPIO toggling in ACPI AML](gpio.md)
+
+## devicetree
+
+- [Adding devices to a device tree](devicetree.md)