doc: develop: manifests: external: add mender-mcu

Introduce documentation for using mender-mcu as an external module in
Zephyr.

Signed-off-by: Daniel Skinstad Drabitzius <[email protected]>

Author: danielskinstad

doc/develop/manifest/external/mender-mcu.rst

@@ -0,0 +1,57 @@
+.. _external_module_mender_mcu:
+
+mender-mcu
+#######################
+
+Introduction
+************
+
+`mender-mcu`_ enables robust firmware updates on resource-constrained devices by integrating with Zephyr. It
+implements an Update Module interface, which allows a module to define the specifics of how to handle an update.
+mender-mcu provides a default Update Module that integrates with MCUboot to provide A/B updates. This allows
+microcontroller units (MCUs) to perform atomic, fail-safe OTA updates with automatic rollback on failure, similar
+to Mender's updates for Linux devices.
+
+The client communicates with the Mender server to report device inventory and identity, checks for
+available updates, downloads new firmware, and coordinates with the Update Module to install updates safely.
+
+mender-mcu is licensed under the Apache-2.0 license.
+
+Requirements
+************
+
+* cJSON for JSON parsing
+
+Usage with Zephyr
+*****************
+
+To pull in mender-mcu as a Zephyr :ref:`module <modules>`, either add it as a West project in the ``west.yaml``
+file or pull it in by adding a submanifest (e.g. ``zephyr/submanifests/mender-mcu.yaml``) file
+with the following content and run ``west update``:
+
+.. code-block:: yaml
+
+   manifest:
+     projects:
+       - name: mender-mcu
+         url: https://github.com/mendersoftware/mender-mcu
+         revision: main
+         path: modules/mender-mcu # adjust the path as needed
+
+For more detailed instructions and API documentation, refer to the `mender-mcu documentation`_.
+The `Zephyr reference project`_ provides an example reference integration.
+
+References
+**********
+
+.. target-notes::
+
+.. _mender-mcu:
+   https://github.com/mendersoftware/mender-mcu
+
+.. _mender-mcu documentation:
+   https://docs.mender.io/operating-system-updates-zephyr
+
+.. _Zephyr reference project:
+   https://github.com/mendersoftware/mender-mcu-integration
+