docs: rename INDEX files to meaningful names and fix navigation

- Renamed subdirectory INDEX.md files to section-specific names:
  - user/INDEX.md → user/user-guide.md
  - developer/INDEX.md → developer/development.md
  - operations/INDEX.md → operations/operations.md
  - contributing/INDEX.md → contributing/contributing.md
  - developer/api/INDEX.md → developer/api/api-reference.md
  - developer/api/collections/INDEX.md → developer/api/collections/collections-api.md

- Updated all internal links to use new filenames
- Updated navigation.go to skip subdirectory INDEX files from sidebar
- Added Dockerfile to include docs directory in container build

This fixes the issue where multiple 'INDEX' links appeared in the sidebar,
making navigation confusing. Now each section has a descriptive name.

docs/user/user-guide.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)