docs: add NVDIMM ACPI documentation

It describes the basic concepts of NVDIMM ACPI and the interfaces
between QEMU and the ACPI BIOS

Signed-off-by: Xiao Guangrong <guangrong.xiao@linux.intel.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
