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

hypervisor (see https://projectacrn.org/).  The process for getting

this to work is somewhat involved, however.

ACRN hypervisor supports a hybrid scenario where Zephyr runs in a so-

called "pre-launched" mode. This means Zephyr will access the ACRN

hypervisor directly without involving the SOS VM. This is the most

practical user scenario in the real world because Zephyr's real-time

and safety capability can be assured without influence from other

VMs. The following figure from ACRN's official documentation shows

how a hybrid scenario works:

.. figure:: ACRN-Hybrid.png

    :align: center

    :alt: ACRN Hybrid User Scenario

    :figclass: align-center

    :width: 80%

    ACRN Hybrid User Scenario

In this tutorial, we will show you how to build a minimal running instance of Zephyr

and ACRN hypervisor to demonstrate that it works successfully. To learn more about

other features of ACRN, such as building and using the SOS VM or other guest VMs,

please refer to the Getting Started Guide for ACRN:

https://projectacrn.github.io/latest/tutorials/using_hybrid_mode_on_nuc.html

Build your Zephyr App

*********************

        west build -b acrn_ehl_crb samples/hello_world

In this tutorial, we will use the Intel Elkhart Lake Reference Board

(`EHL`_ CRB) since it is one of the suggested platform for this

type of scenario. Use ``acrn_ehl_crb`` as the target board parameter.

Note the kconfig output in ``build/zephyr/.config``, you will need to

reference that to configure ACRN later.

        git clone https://github.com/projectacrn/acrn-hypervisor

We suggest that you use versions v2.5.1 or later of the ACRN hypervisor

as they have better support for SMP in Zephyr.

Like Zephyr, ACRN favors build-time configuration management instead

of runtime probing or control.  Unlike Zephyr, ACRN has single large

configuration files instead of small easily-merged configuration

    .. code-block:: xml

        <vm id="0">

            <vm_type>SAFETY_VM</vm_type>

            <name>ACRN PRE-LAUNCHED VM0</name>

            <guest_flags>

                <guest_flag>0</guest_flag>

            </guest_flags>

            <cpu_affinity>

                <pcpu_id>0</pcpu_id>

                <pcpu_id>1</pcpu_id>

            </cpu_affinity>

            ...

            <clos>

                <vcpu_clos>0</vcpu_clos>

                <vcpu_clos>0</vcpu_clos>

            </clos>

            ...

        </vm>

To use SMP, we have to change the pcpu_id of VM0 to 0 and 1.

This configures ACRN to run Zephyr on CPU0 and CPU1. The ACRN hypervisor

and Zephyr application will not boot successfully without this change.

If you plan to run Zephyr with one CPU only, you can skip it.

Since Zephyr is using CPU0 and CPU1, we also have to change

VM1's configuration so it runs on CPU2 and CPU3. If your ACRN set up has

additional VMs, you should change their configurations as well.

    .. code-block:: xml

        <vm id="1">

            <vm_type>SOS_VM</vm_type>

            <name>ACRN SOS VM</name>

            <guest_flags>

                <guest_flag>0</guest_flag>

            </guest_flags>

            <cpu_affinity>

                <pcpu_id>2</pcpu_id>

                <pcpu_id>3</pcpu_id>

            </cpu_affinity>

            <clos>

                <vcpu_clos>0</vcpu_clos>

                <vcpu_clos>0</vcpu_clos>

            </clos>

            ...

        </vm>

Note that these indexes are physical CPUs on the host.  When

configuring multiple guests, you probably don't want to overlap these

        ----- Entering VM 0 Shell -----

        *** Booting Zephyr OS build v2.6.0-rc1-324-g1a03783861ad  ***

        Hello World! acrn

.. _EHL: https://www.intel.com/content/www/us/en/products/docs/processors/embedded/enhanced-for-iot-platform-brief.html