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