docs: add comprehensive WebView widget documentation (beta)

- Add docs/WebView.md with full API reference, examples, JS bridge API,
  platform notes, error handling, debug mode, and size hints
- Update sidebar with WebView section flagged as Beta
- Update architecture.md with WebView helper process model, IPC protocol,
  and comparison table vs Tcl/Tk widgets

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: developersharif