refactor: make content educational instead of promotional

- Add HTML element code to section titles (e.g., '<dialog>', '<progress>')
- Replace marketing badges (COMPLEX, ZERO-JS) with neutral JS/HTML labels
- Simplify column titles to 'JavaScript' and 'Native HTML'
- Update JS autocomplete example to use cities (matching native side)
- Fix input styling in JS column to use gray borders/outlines
- Simplify summary section text

Author: nextlevelshit