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

[codezerro]

📓 Description:

Create and publish the web documentation for the newly developed Deep Learning-based Digit Classification exercise in Robotics Academy. This documentation should guide users through uploading their ONNX models, using the code editor for inference, and understanding the webcam-based digit recognition workflow.

✅ Checklist:

• [x] Add updated description of the exercise
• [x] Document how to upload and use custom ONNX models
• [x] Provide example inference code with preprocessing steps
• [x] Explain the input/output format requirements for ONNX models
• [ ] Add UI screenshots or video preview
• [x] Add references link
• [ ] Review and finalize formatting, grammar, and links