Move Pro files (Step 8)

Moved 1 file to pro/:
- react-on-rails-pro/react-on-rails-pro.md

Note: major-performance-breakthroughs-upgrade-guide.md was already moved
to upgrading/ in Step 6 (it's an upgrade guide, not Pro features doc).

Kept old react-on-rails-pro/ folder for reference (will clean up later).

Author: ihabadham