doc: add Edge Impulse SDK external module documentation

Add documentation for the Edge Impulse SDK Zephyr module to the external
modules section, including:

- Module overview describing the Edge Impulse ML development platform
- Integration instructions with west.yml manifest configuration
- West extension commands (ei-build, ei-deploy) for model deployment
- License information (BSD-3-Clause-Clear)
- References to official Edge Impulse documentation and tutorials

The Edge Impulse SDK module enables machine learning inference on Zephyr
devices by packaging the Edge Impulse SDK as a reusable Zephyr module
with west integration for streamlined ML model deployment workflows.

Signed-off-by: Eoin Jordan <eoin@edgeimpulse.com>