boards: nrf54h20: extend and fix documentation

  Extensions, running at up to 320 MHz, referred to as the **application core**

* an Arm Cortex-M33 core with DSP instructions, FPU, and Armv8-M Security

  Extensions, running at up to 256 MHz, referred to as the **radio core**.

* a Nordic VPR RISC-V core, referred to as the **ppr core** (Peripheral

  Processor).

The ``nrf54h20dk/nrf54h20/cpuapp`` build target provides support for

the application core on the nRF54H20 SoC.

The ``nrf54h20dk/nrf54h20/cpurad`` build target provides support for

the radio core on the nRF54H20 SoC.

The ``nrf54h20dk/nrf54h20/cpuppr`` build target provides support for

the PPR core on the nRF54H20 SoC executing from RAM.

The ``nrf54h20dk/nrf54h20/cpuppr/xip`` build target provides support for

the PPR core on the nRF54H20 SoC executing from MRAM.

nRF54H20 SoC provides support for the following devices:

Programming and Debugging

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

Applications for both the ``nrf54h20dk/nrf54h20/cpuapp`` and

``nrf54h20dk/nrf54h20/cpurad`` targets can be built, flashed,

and debugged in the usual way. See :ref:`build_an_application`

and :ref:`application_run` for more details on building and running.

Applications for all targets can be built and flashed the usual way.

See :ref:`build_an_application` and :ref:`application_run` for more details on

building and running. Debugging is for now limited to the application and radio

cores only, using :ref:`nordic_segger`.

Flashing

========

As an example, this section shows how to build and flash the :ref:`hello_world`

application.

Follow the instructions in the :ref:`nordic_segger` page to install

and configure all the necessary software. Further information can be

found in :ref:`nordic_segger_flashing`.

To build and program the sample to the nRF54H20 DK, complete the following steps:

1. Connect the nRF54H20 DK to your computer using the IMCU USB port on the DK.

2. Install `nRF Util`_

#. Build the sample by running the following command:

   .. zephyr-app-commands::

You can build and flash the examples to make sure Zephyr is running correctly on

your board. The button and LED definitions can be found in

:zephyr_file:`boards/nordic/nrf54h20dk/nrf54h20dk_nrf54h20_cpuapp.dts`.

.. _nRF Util:

   https://www.nordicsemi.com/Products/Development-tools/nrf-util