docs: rewrite README for first-time users

Restructure README following Doumont's communication principles to
create a better first-impression experience for developers new to
the framework.

Key changes:
- Lead with working code example (25 lines) instead of prose
- Add clear value proposition: "Concurrent Rust made simple"
- Add "When to Use Acton" section for honest self-selection guidance
- Organize core concepts in scannable table format
- Reduce from 246 to 202 lines while improving information density
- Preserve sponsor section and license info

Author: rrrodzilla