docs: Add comprehensive invoice system implementation guide

- Document various invoice generation approaches (PDF, HTML-to-PDF, Stripe)
- Compare Stripe Payment Intents vs Invoicing vs Checkout Sessions
- Provide complete code implementation with models, controllers, services
- Include phase-by-phase implementation strategy for current use case
- Add testing, security, and deployment guidelines
- Recommend hybrid approach: keep current checkout + post-payment invoices

🤖 Generated with [Claude Code](https://claude.ai/code)

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

app/views/events/index.html.erb

@@ -6,6 +6,7 @@
       événements trouvés
     </div>
   </div>
+
   <!-- Breadcrumb -->
   <nav class="mb-6" aria-label="Breadcrumb">
     <ol class="flex items-center space-x-2 text-sm">