Documentation/virt/kvm: Document on Trust Domain Extensions (TDX)

Add documentation to Intel Trusted Domain Extensions (TDX) support.

Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
Signed-off-by: Binbin Wu <binbin.wu@linux.intel.com>
Message-ID: <20250227012021.1778144-21-binbin.wu@linux.intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>