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/contributing/contributing.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)