doc: add mmio-dev-passthrough to TOC
Doc was merged but not included in the TOC (CI indicated a pass on that
PR even though doc build failed).  This fixes that undetected error.
@@ -18,6 +18,7 @@ Hypervisor high-level design
Virtual Interrupt <hv-virt-interrupt>
VT-d <hv-vt-d>
Device Passthrough <hv-dev-passthrough>
+ mmio-dev-passthrough
hv-partitionmode
Power Management <hv-pm>
Console, Shell, and vUART <hv-console>
