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

[danielskinstad]

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

[github-actions]

Hello @danielskinstad, and thank you very much for your first pull request to the Zephyr project!
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[danielskinstad]

🏓

[henrikbrixandersen]

The client is open source under Apache-2.0, but what about the server side? Is it open source, closed source, a paid service, or something else? I think it would be good to at least mention a few details on the server side here as well.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[github-actions]

Hi @danielskinstad!
Congratulations on getting your very first Zephyr pull request merged 🎉🥳. This is a fantastic achievement, and we're thrilled to have you as part of our community!

To celebrate this milestone and showcase your contribution, we'd love to award you the Zephyr Technical Contributor badge. If you're interested, please claim your badge by filling out this form: Claim Your Zephyr Badge.

Thank you for your valuable input, and we look forward to seeing more of your contributions in the future! 🪁