Add integration guide and standalone OpenHD MAVLink helper

[raphaelscholle]

Summary

• move the external OpenHD MAVLink guide into a new top-level integration folder and extend it with helper usage examples
• add a standalone C++ CLI that can issue the documented OpenHD link, scan, video, camera, and recording commands over MAVLink
• document how to build and invoke the helper for external integrations

Testing

• g++ -std=c++17 -Ilib/mavlink-headers -Ilib/mavlink-headers/mavlink/v2.0 integration/openhd_mavlink_tool.cpp -o integration/openhd_mavlink_tool
• ./integration/openhd_mavlink_tool --help

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Codex Task