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/contributing/INDEX.md

@@ -0,0 +1,35 @@
+# Contributing to Bookhoard
+
+Welcome to the Bookhoard contributing documentation. This section contains guides for contributing to the Bookhoard project.
+
+## 🚀 Getting Started
+
+- **[Development Guide](DEVELOPMENT.md)** - Development workflow and architecture
+  - Architecture overview
+  - Directory structure
+  - Local development setup
+  - Testing guidelines
+  - Code style
+  - Deployment
+
+## 📋 Project Guidelines
+
+- **[PROJECT_GUIDELINES.md](../PROJECT_GUIDELINES.md)** - Development rules and standards
+  - Critical prohibitions
+  - Mandatory requirements
+  - Error recovery protocol
+  - Workflow checklists
+
+## 🤝 How to Contribute
+
+We welcome contributions! Please see our [Development Guide](DEVELOPMENT.md) for information on:
+- Setting up your development environment
+- Understanding the codebase
+- Making pull requests
+- Code review process
+- Testing requirements
+
+---
+
+**Looking for user documentation?** See the [User Portal](../user/INDEX.md)
+**Need API docs?** See the [Developer Portal](../developer/INDEX.md)