docs: restructure README for 5-second value proposition (#126)

- Add badges (build, license, Java 11+, stars)
- New punchy tagline: "One CLI for all JVM diagnostics"
- Comparison table vs Arthas/VisualVM/jcmd/GCEasy up front
- 4 key feature demos: doctor, gclog, watch, tui
- Compact grouped command tables (all 55 commands)
- Fix "50 commands" → "55 commands"
- Reduced from 528 → 465 lines while keeping all content

Signed-off-by: rlaope <piyrw9754@gmail.com>

Author: rlaope