fix: add new documentation pages to mkdocs navigation

Added three new documentation pages to mkdocs.yml navigation:

- Features → Vendor Landing Pages (features/vendor-landing-pages.md)
- Frontend Development → Shop Frontend → Navigation Flow (frontend/shop/navigation-flow.md)
- Troubleshooting → Shop Frontend (troubleshooting/shop-frontend.md)

This fixes the MkDocs build warnings about orphaned pages not in navigation.

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

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

mkdocs.yml

@@ -92,6 +92,7 @@ nav:
     - Shop Frontend:
       - Architecture: frontend/shop/architecture.md
       - Page Templates: frontend/shop/page-templates.md
+      - Navigation Flow: frontend/shop/navigation-flow.md
 
   # ============================================
   # DEVELOPMENT (Shared Development Resources)
@@ -144,6 +145,7 @@ nav:
     - Content Management System:
       - Overview: features/content-management-system.md
       - Implementation Guide: features/cms-implementation-guide.md
+    - Vendor Landing Pages: features/vendor-landing-pages.md
 
   # ============================================
   # USER GUIDES
@@ -155,6 +157,12 @@ nav:
     - CSV Import: guides/csv-import.md
     - Marketplace Integration: guides/marketplace-integration.md
 
+  # ============================================
+  # TROUBLESHOOTING
+  # ============================================
+  - Troubleshooting:
+    - Shop Frontend: troubleshooting/shop-frontend.md
+
 # Theme configuration
 theme:
   name: material