Move major-performance-breakthroughs-upgrade-guide to pro/

This file is a Pro features announcement (React Server Components, SSR
Streaming, Early Hydration), not upgrade instructions. Better suited
for pro/ folder as it showcases Pro v4 capabilities.

upgrading/ now has 3 files:
- upgrading-react-on-rails.md (upgrade HOW-TO)
- release-notes/ folder (2 files)

pro/ now has 2 files:
- react-on-rails-pro.md (Pro overview)
- major-performance-breakthroughs-upgrade-guide.md (Pro features)

Author: ihabadham