Add compatibility links to main site navigation

- Add "Compatibility" link under Hardware dropdown in site header
- Add "Check compatibility" CTA on /hardware/ page (hero + bottom)
- Redirect old /compatibility/ page to compat.reglinux.org
- Delete dead _data/compatibility_matrix.yml (superseded by D1)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: rtissera