Improve Documentation: Examples, Concepts, Getting Started, Supported Formats, and Index

Requested feature

Comprehensive documentation improvements to enhance user experience and onboarding for new Docling users.
This update addresses the need for clearer examples, structured learning paths, and professional documentation that scales for enterprise adoption.

Key additions:
• Examples Index: Expanded into a full guide with categorized examples and suggested learning paths.
• Concepts Index: Enhanced to include a complete overview of system concepts, architecture, and learning progression.
• Getting Started Guide: Rewritten as a comprehensive, step-by-step tutorial to reduce onboarding friction.
• Supported Formats: Extended with detailed descriptions of all supported formats, their capabilities, and use cases.
• Main Index: Cleaned up and streamlined to present features in a more professional and enterprise-ready style.

Alternatives
• Continue using the old, minimal documentation (2–17 line stubs) which lacks sufficient guidance for new users.
• Rely on external tutorials or community discussions for onboarding, which creates inconsistency and slows adoption.
• Incremental updates per section (piecemeal), but this would not provide the unified experience or professional polish needed for enterprise readiness.