Documentation: hyperv: Add overview of Hyper-V enlightenments

Add an initial documentation topic for Linux enlightenments to
run as a guest on Microsoft's Hyper-V hypervisor, linked under
the "virt" documentation area. Update the virt doc index.rst
and the MAINTAINERS file.

Signed-off-by: Michael Kelley <mikelley@microsoft.com>
Link: https://lore.kernel.org/r/1657561704-12631-2-git-send-email-mikelley@microsoft.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
