doc: develop: manifests: external: add CANnectivity USB to CAN adapter

Add a reference to the CANnectivity USB to CAN adapter firmware as an
external module.

Signed-off-by: Henrik Brix Andersen <[email protected]>

doc/develop/manifest/external/cannectivity.rst

@@ -0,0 +1,57 @@
+.. _external_module_cannectivity:
+
+CANnectivity USB to CAN adapter firmware
+########################################
+
+Introduction
+************
+
+`CANnectivity`_ is an open source firmware for Universal Serial Bus (USB) to Controller Area Network
+(CAN) adapters.
+
+The firmware implements the Geschwister Schneider USB/CAN device protocol (often referred to as
+"gs_usb"). This protocol is supported by the Linux kernel SocketCAN `gs_usb driver`_, by
+`python-can`_, and by many other software packages.
+
+The firmware, which is based on Zephyr RTOS, allows turning your favorite microcontroller
+development board into a full-fledged USB to CAN adapter.
+
+CANnectivity is licensed under the Apache-2.0 license.
+
+Usage with Zephyr
+*****************
+
+The CANnectivity firmware repository is a Zephyr :ref:`module <modules>` which allows for reuse of
+its components (i.e. the "gs_usb" protocol implementation) outside of the CANnectivity firmware
+application.
+
+To pull in CANnectivity as a Zephyr module, either add it as a West project in the ``west.yaml``
+file or pull it in by adding a submanifest (e.g. ``zephyr/submanifests/cannectivity.yaml``) file
+with the following content and run ``west update``:
+
+.. code-block:: yaml
+
+   manifest:
+     projects:
+       - name: cannectivity
+         url: https://github.com/CANnectivity/cannectivity.git
+         revision: main
+         path: custom/cannectivity # adjust the path as needed
+
+Once CANnectivity is added as a Zephyr module, the "gs_usb" implementation can be reused outside of
+the CANnectivity firmware application by including its header:
+
+.. code-block:: c
+
+   #include <cannectivity/usb/class/gs_usb.h>
+
+Please see the header file for the API details.
+
+.. _CANnectivity:
+   https://github.com/CANnectivity/cannectivity
+
+.. _gs_usb driver:
+   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/net/can/usb/gs_usb.c
+
+.. _python-can:
+   https://python-can.readthedocs.io/en/stable/interfaces/gs_usb.html