docs: acrn: update the documentation of setting the ACRN hypervisor

Add some descriptions of hybrid scenario of ACRN hypervisor, and
completed the configurations that we are using to build ACRN. This
configuration change for ACRN hypervisor is necessary when our Zephyr
application is using over than one CPU for it.

Signed-off-by: Enjia Mai <enjia.mai@intel.com>