Record and add WebGPU demo GIF

[io-eric]

Task

The README currently has animated previews for Canvas, WebGL, and DOM, but the WebGPU section is missing its GIF. We need a visual demo to complete the gallery.

Requirements

• Frame Rate: 30 FPS.
• Resolution: 400px wide (to match existing GIFs).
• Source: Run the code in examples/webgpu/.
• File Path: Save as docs/images/webgpu_demo.gif.

Steps for Contributor

1. Build the toolchain: ./build.sh
2. Compile the WebGPU demo: ./webcc examples/webgpu/main.cc
3. Record the browser window (using a tool like ScreenToGif, GIPHY Capture, or ffmpeg).
4. Update the README.md to display the image.

Note: Ensure you are using a browser that supports WebGPU (Chrome/Edge/Nightly) to record the capture!