docs: restructure README with concise overview and improved organization
- Condensed README from 715 to 113 lines with streamlined feature descriptions
- Replaced verbose documentation with quick-start focused content
- Reorganized sections: removed detailed architecture, API endpoints, and Meilisearch implementation details
- Added version badges and simplified feature categories (Navigation, Markdown, Graph)
- Streamlined installation instructions with Docker-first approach and simplified local
- Integrated Unsplash API for image search functionality with environment configuration
- Added new Nimbus Editor page component with navigation from sidebar and mobile drawer
- Enhanced TOC with highlight animation for editor heading navigation
- Improved CDK overlay z-index hierarchy for proper menu layering
- Removed obsolete logging validation script
