Docs: Demo GIF

The README has a plain text demo block. A real GIF would make it much more attractive.

What we need

A recording of:

1. Running npx vscode-setup
2. Picking 1-2 categories
3. Selecting extensions
4. Seeing install progress

Free tools

• asciinema.org - records and shares as animated SVG
• terminalizer.com - exports to GIF
• github.com/charmbracelet/vhs - script your recording

How to contribute

1. Record the demo
2. Add to docs/assets/demo.gif
3. Replace the text demo section in README.md with ![Demo](docs/assets/demo.gif)
4. Open a PR

No coding required!