Documentation: hyperv: Add overview of Confidential Computing VM support

Add documentation topic for Confidential Computing (CoCo) VM support
in Linux guests on Hyper-V.
Signed-off-by: Michael Kelley <mhklinux@outlook.com>
Reviewed-by: Easwar Hariharan <eahariha@linux.microsoft.com>
Link: https://lore.kernel.org/r/20240618165059.10174-1-mhklinux@outlook.comSigned-off-by: Wei Liu <wei.liu@kernel.org>
Message-ID: <20240618165059.10174-1-mhklinux@outlook.com>

Documentation/virt/hyperv/index.rst

@@ -11,3 +11,4 @@ Hyper-V Enlightenments
    vmbus
    clocks
    vpci
+   coco