Update Theremin Simulator documentation

[91volt]

Summary
Refresh the Theremin Simulator README to match the current behavior of the WaveGenerator Brick and the updated web interface, and replace the screenshot asset.

What changed

• Clarified that audio synthesis is handled by the WaveGenerator Brick, including smoothing, envelope and glide, instead of custom manual audio loops.
• Updated the description of user interaction: power switch, master volume control, play area mapping for pitch and volume, and grid toggle.
• Refined hardware requirements and compatibility notes for USB audio devices, making it clear that HDMI audio and Bluetooth speakers are not supported.
• Expanded the How it works section with a clearer high level data flow and a more accurate description of the backend and frontend roles.
• Simplified and aligned troubleshooting and technical details (sample rate, envelope parameters, update rate) with the current implementation.
• Replaced the Theremin Simulator screenshot asset to match the new interface and optimise image size.