docs: add Letzshop Admin Management guide

New comprehensive guide covering:
- Products tab (viewing, import, export)
- Orders tab overview
- Exceptions tab overview
- Jobs tab (all job types: import, export, historical, sync)
- Settings tab configuration
- API endpoints reference
- Best practices and troubleshooting

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

mkdocs.yml

@@ -198,6 +198,7 @@ nav:
     - CSV Import: guides/csv-import.md
     - Marketplace Integration: guides/marketplace-integration.md
     - Letzshop:
+      - Admin Management: guides/letzshop-admin-management.md
       - Order Integration: guides/letzshop-order-integration.md
       - Marketplace API: guides/letzshop-marketplace-api.md