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/user/INDEX.md

@@ -0,0 +1,52 @@
+# User Documentation
+
+Welcome to the Bookhoard user documentation. This section contains guides for using Bookhoard features, setting up devices, and configuring sync.
+
+## 📱 Device Setup
+
+Learn how to configure your e-reader devices to sync with Bookhoard:
+
+- **[Kobo Setup Guide](devices/kobo-setup.md)** - Complete guide for Kobo e-readers
+  - Device registration
+  - Sync configuration
+  - OPDS wireless book delivery
+  - Troubleshooting
+
+- **[KOReader Setup Guide](devices/koreader-setup.md)** - Complete guide for KOReader
+  - Installation on Kindle/Kobo/PocketBook
+  - Sync setup
+  - OPDS catalog access
+  - Troubleshooting
+
+## 🔄 Sync Configuration
+
+- **[Universal Sync Guide](sync-guide.md)** - Understanding and using sync features
+  - What is Universal Sync?
+  - Supported devices
+  - Book matching and auto-linking
+  - Conflict resolution
+  - Best practices
+
+## 🎨 Frontend Guide
+
+**[Frontend Guide](frontend-guide.md)** - Learn how to use the Bookhoard web interface
+- *Coming Soon*
+
+## 👤 User Areas
+
+**[User Areas Guide](user-areas.md)** - Managing your personal library and settings
+- *Coming Soon*
+
+## ⚙️ Settings
+
+**[Settings Guide](settings-guide.md)** - Configuring your Bookhoard preferences
+- *Coming Soon*
+
+## 🔐 Admin Features
+
+**[Admin Guide](admin-guide.md)** - Administrative functions and management
+- *Coming Soon*
+
+---
+
+**Looking for developer documentation?** See the [Developer Portal](../developer/INDEX.md)