doc: develop: manifests: external: add libmpix

Introduce documentation for using libmpix as an external module in
Zephyr.

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>

doc/develop/manifest/external/libmpix.rst

@@ -0,0 +1,59 @@
+.. _external_module_libmpix:
+
+libmpix
+#######
+
+Introduction
+************
+
+The `libmpix`_ project provides a library for working with image data on microcontrollers.
+It supports pixel format conversion, debayer, blur, sharpen, color correction, resizing and more.
+
+It pipelines multiple operations together, eliminating intermediate buffers.
+This allows larger image resolutions to fit in constrained systems without compromising performance.
+
+Features
+********
+
+* Simple zero-copy, pipelined engine with low runtime overhead
+* Reduces memory overhead (for example processes 1 MB of data with only 5 kB of RAM)
+* POSIX support (Linux/BSD/MacOS) and Zephyr support
+
+Usage with Zephyr
+*****************
+
+To pull in libmpix as a Zephyr module, either add it as a West project in the :file:`west.yaml`
+file or pull it in by adding a submanifest (e.g. ``zephyr/submanifests/libmpix.yaml``) file
+with the following content and run :command:`west update`:
+
+.. code-block:: yaml
+
+   manifest:
+     projects:
+       - name: libmpix
+         url: https://github.com/libmpix/libmpix.git
+         revision: main
+         path: modules/lib/libmpix
+
+Refer to the ``libmpix`` headers for API details. A brief example is shown below.
+
+.. code-block:: c
+
+   #include <mpix/image.h>
+
+   struct mpix_image img;
+
+   mpix_image_from_buf(&img, buf_in, sizeof(buf_in), MPIX_FORMAT_RGB24);
+   mpix_image_kernel(&img, MPIX_KERNEL_DENOISE, 5);
+   mpix_image_kernel(&img, MPIX_KERNEL_SHARPEN, 3);
+   mpix_image_convert(&img, MPIX_FORMAT_YUYV);
+   mpix_image_to_buf(&img, buf_out, sizeof(buf_out));
+
+   return img.err;
+
+References
+**********
+
+.. target-notes::
+
+.. _libmpix: https://github.com/libmpix/libmpix