feat: cli upgrade/remove/uninstall commands + cleaned-up docs page

CLI:
- c0upons upgrade (alias: update) — re-downloads and replaces self
- c0upons remove (alias: uninstall) — deletes binary from PATH

Docs:
- Cleaner layout with bordered endpoint cards and coloured method badges
- New commands documented (upgrade, remove)
- Tighter sidebar nav, better table styling, remove/uninstall section

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: ralyodio