[gh-pages] RA: Web documentation for the new "deep learning-based human detection exercise"

[codezerro]

📔 Summary: This PR adds the official web documentation for the new deep learning-based human detection exercise under RoboticsAcademy. The documentation is designed to help users understand how to upload their own ONNX models, write inference code, and detect humans from live webcam input.

Closes #3139