Documentation: Add and link the arch directory

Fixes: b159d5ba8f ("riscv: add documentation for stages and payloads")
Change-Id: I5ca8ed094c9b6d115da707375205872e782a66b2
Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Reviewed-on: https://review.coreboot.org/28991
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Martin Roth <martinroth@google.com>

Documentation/arch/index.md

@@ -0,0 +1,8 @@
+# Architecture-specific documentation
+
+This section contains documentation about coreboot on specific CPU
+architectures.
+
+## RISC-V
+
+- [RISC-V documentation](riscv/index.md)

Documentation/index.md

@@ -15,6 +15,7 @@ Contents:
 * [ABI data consumption](abi-data-consumption.md)
 * [GPIO toggling in ACPI AML](acpi/gpio.md)
 * [Native Graphics Initialization with libgfxinit](gfx/libgfxinit.md)
+* [Architecture-specific documentation](arch/index.md)
 * [Northbridge-specific documentation](northbridge/index.md)
 * [System on Chip-specific documentation](soc/index.md)
 * [Mainboard-specific documentation](mainboard/index.md)