modbus-testing: recipe and modbus Oniro docs

Adding a recipe for our libmodbus usage examples and provide docs on the Modbus integration in Oniro in the supported technologies docs section. Please see the commit messages for details.

docs/supported-technologies/modbus.rst

+.. SPDX-FileCopyrightText: Huawei Inc.
+..
+.. SPDX-License-Identifier: CC-BY-4.0
+
+.. include:: ../definitions.rst
+
+ModBus
+######
+
+`Modbus <https://modbus.org/>` is a communication protocol from the 1970s
+designed to support industrial use cases. Its main modes of operation are Modbus
+RTU (Remote Terminal Unit) and Modbus TCP. The RTU variant is mostly used over
+RS-485 serial ports, while the newer Modbus TCP variant allows for a communication
+over Ethernet for more flexibility.
+
+Besides its core industrial use cases in SCADA (Supervisory control and data
+acquisition) systems it also moved closer to the consumer when being integrated
+into homes e.g. in home heating systems and PV systems.
+
+|main_project_name| maintains support for Modbus in its Linux and Zephyr
+flavours. On the Linux side the user-space Modbus implementation `libmodbus
+<https://libmodbus.org/>` is used while on Zephyr the native `Modbus subsystem
+<https://docs.zephyrproject.org/latest/services/modbus/index.html>` is used.
+
+RTU and TCP operation modes are supported.
+
+.. note::
+
+    Modbus support in the |main_project_name| exists purely for interoperability
+    purpose with existing systems. For new products without this requirement we
+    do not recommend using Modbus.
+
+Enabling Modbus in Oniro:
+*************************
+
+Modbus functionality is not enabled by default in the |main_project_name| and
+there is currently no Blueprint demonstrating its use cases. The basic integration
+is working none the less and Modbus can be enabled in the Linux images by adding
+libmodbus to the image you are building. This can be done by adding the
+following line to your `conf/local.conf` file:
+
+.. code-block::
+
+    IMAGE_INSTALL:append = " libmodbus"
+
+On Zephyr you would need to enable the `CONFIG_MODBUS` options for your modes of
+operations. The Zephyr upstream project does offer `samples for RTU as well as
+TCP operations modes in different roles
+<https://github.com/zephyrproject-rtos/zephyr/tree/main/samples/subsys/modbus>`
+
+Modbus-testing:
+***************
+
+The `libmodbus` library does not come with any examples on how to use it by
+default. To fill this gap and allow integrators and developers to have a
+head-start when developing an application using `libmodbus` we developed a small
+set of examples to demonstrate the usage. These examples can be found at our
+`modbus-testing repo <https://gitlab.eclipse.org/stefanschmidt/modbus-testing>`
+and included in the image by the following addition to your `conf/local.conf` file:
+
+.. code-block::
+
+    IMAGE_INSTALL:append = " libmodbus modbus-testing"
+
+To test Modbus RTU communication over an existing RS-485 serial port (e.g.
+/dev/ttyUSB0) under Linux you can run a server as follows:
+
+.. code-block:: console
+
+    $ modbus-server /dev/ttyUSB0
+
+To execute the client side instead run the following command:
+
+.. code-block:: console
+
+    $ modbus-client /dev/ttyUSB0
+
+To run the same example for Modbus TCP instead of RTU adapt the parameters as
+shown below:
+
+.. code-block:: console
+
+    $ modbus-server -tcp
+
+.. code-block:: console
+
+    $ modbus-client -tcp SERVER_IP_ADDRESS_HERE
+
+Keep in mind that these examples have hard-coded values for many parts (e.g.
+device id, register values, etc) as they are only suited for demonstration
+purpose and as a head-start for your own development.
+