boards/x86/acrn: Rework board documentation

ACRN build and configuration is non-trivially complicated, and so far
integration documentation has been mostly missing, and users have had
to get by via copying from existing integration efforts with minor
changes, leading to repeated mistakes and persistent confusion.  This
is an attempt to document the process from first principles, with an
eye toward informing integrators (not me!) who might come by later to
better automate things.  Some of the content is going to look remedial
to someone already familiar with e.g. ACRN configuration or EFI boot.

This simply replaces the pre-existing docs, which were for earlier
versions of ACRN where Zephyr was launched from the service OS instead
of the now-standard pre-launch VM mode.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>