docs: powerpc: Document nested KVM on POWER

Document support for nested KVM on POWER using the existing API as well
as the new PAPR API. This includes the new HCALL interface and how it
used by KVM.

Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Jordan Niethe <jniethe5@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230914030600.16993-12-jniethe5@gmail.com

Author: mikey

Documentation/powerpc/index.rst

@@ -26,6 +26,7 @@ powerpc
     isa-versions
     kaslr-booke32
     mpc52xx
+    kvm-nested
     papr_hcalls
     pci_iov_resource_on_powernv
     pmu-ebb