docs: update end user documentation

[marc0olo]

This PR represents a complete overhaul of the icp-cli documentation, transforming it into a professional, framework-driven system that serves users at every stage of their journey.

Note:

• There might still be issues and the docs have not yet been carefully reviewed. Most of the documentation was created with the support of Claude.

What Changed

Implemented Diátaxis Documentation Framework

Reorganized documentation into four distinct categories following industry best practices:

• Tutorial - Learning-oriented first deployment guide
• Guides - Task-oriented how-to instructions for common workflows
• Concepts - Understanding-oriented explanations of architecture
• Reference - Information-oriented technical specifications

New Content Added (~3,000 lines)

• Concepts: Project model, environments, build-deploy-sync pipeline, recipes architecture
• Guides: Local development, mainnet deployment, identity management, environment configuration, recipe/template creation, tokens & cycles, installation
• Migration: Comprehensive dfx-to-icp-cli migration guide with command mappings, project conversion examples, and identity import instructions
• Reference: Complete configuration reference, canister settings documentation

Improved Navigation

• Added central documentation hub docs/index.md
• Created category index pages for easy navigation
• Updated README.md with clear documentation links
• Moved CONTRIBUTING.md to project root

Content Cleanup

• Removed outdated files: getting-started.md, project-configuration.md, workflows.md
• Consolidated overlapping content into focused guides
• Streamlined verbose sections