Update Theremin Simulator example documentation

[91volt]

Summary

This PR updates the documentation for the Theremin Simulator example to align with the latest implementation of the wave_generator Brick and the updated web interface.

The Theremin Simulator demonstrates how to create a virtual instrument using real-time audio synthesis. It captures user input via a web interface and streams synthesized audio to a USB speaker, utilizing the wave_generator Brick for envelope smoothing and low-latency output.

What changed

• Updated README.md for Theremin Simulator:
  • Clarified that audio synthesis—including smoothing, envelope, and glide—is handled automatically by the wave_generator Brick, replacing manual audio loops.
  • Updated the User Interaction section to describe the new controls: Power switch, Master Volume, and Grid toggle.
  • Refined the Hardware Requirements to explicitly state that a USB-C hub and USB audio device are mandatory, while HDMI and Bluetooth audio are not supported.
  • Expanded the "How it Works" section with a high-level data flow diagram and detailed descriptions of the backend and frontend roles.
  • Aligned the Troubleshooting and Technical Details sections (sample rate, envelope parameters) with the current code implementation.
• Asset Updates:
  • Replaced the example screenshot to reflect the new user interface design and optimized the image size.
• License Compliance:
  • Added SPDX license headers to needed code files to pass repository compliance checks.