Documentation: hyperv: Add overview of VMbus

Add documentation topic for using VMbus when running as a guest
on Hyper-V.

Signed-off-by: Michael Kelley <mikelley@microsoft.com>
Link: https://lore.kernel.org/r/1657561704-12631-3-git-send-email-mikelley@microsoft.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>