docs: restructure documentation into audience-based portals

BREAKING CHANGE: Documentation URLs have changed

New structure:
- user/ - End-user documentation (device setup, sync guides, frontend)
- developer/ - Developer documentation (API reference, protocols, specs)
- operations/ - Operations documentation (deployment, troubleshooting)
- contributing/ - Contribution guides

Changes:
- Created portal INDEX.md files for each audience section
- Moved device guides to user/devices/ (kobo-setup.md, koreader-setup.md)
- Moved API docs to developer/ (api-reference.md, collections-api.md)
- Moved sync guide to user/sync-guide.md
- Moved troubleshooting to operations/troubleshooting.md
- Moved all split API docs to developer/api/
- Renamed protocol files (kobo-protocol.md, koreader-protocol.md)
- Added placeholder user guides (frontend, user-areas, settings, admin)
- Updated all internal links to new paths
- Updated Go code (http_handler.go, navigation.go) for new paths
- Updated main INDEX.md for audience-based navigation

Benefits:
- Clear separation of user and developer documentation
- Scalable structure for future user guide expansion
- Better organization and discoverability
- Audience-specific landing pages

Related to DOCS_IMPLEMENTATION_PLAN.md Phase 2 completion

docs/operations/INDEX.md

@@ -0,0 +1,34 @@
+# Operations Documentation
+
+Welcome to the Bookhoard operations documentation. This section contains guides for deploying, maintaining, and troubleshooting Bookhoard instances.
+
+## 🔧 Deployment
+
+### Quick Start
+
+1. **[Troubleshooting Guide](troubleshooting.md)** - Common deployment issues and solutions
+   - Environment variables setup
+   - Port conflicts
+   - Container runtime (Podman)
+   - Network configuration
+   - Performance optimization
+
+### Additional Deployment Guides
+
+**[Deployment Guide](deployment.md)** - Comprehensive deployment guide
+- *Coming Soon*
+
+## 🛠️ Maintenance
+
+**[Maintenance Guide](maintenance.md)** - Ongoing operations and maintenance
+- *Coming Soon*
+
+## 📊 Monitoring
+
+**[Monitoring Guide](monitoring.md)** - Monitoring and alerting
+- *Coming Soon*
+
+---
+
+**Looking for user documentation?** See the [User Portal](../user/INDEX.md)
+**Need API docs?** See the [Developer Portal](../developer/INDEX.md)