Docs: restructure docs for better readability

[Harshal6927]

Summary

• Structure the documentation in a more LLM-friendly format, preferably using Markdown for better readability and parsing
• Create llms.txt and llms-full.txt
• Add live playground. Reference: feat: add interactive Pyodide playground for SQLSpec documentation sqlspec#243

Proof read, validation and fixes

• Modeling
  • Basics
  • Relationships
  • Advanced
• Repositories
  • Basics
  • Filtering
  • Advanced
• Services
  • Basics
  • Schema integration
  • Advanced
• Custom Types
  • Basic
  • Security
  • File
  • Advanced
• Storage Backends
  • FSSpec
  • Obstore
  • Storage configuration
• CLI
  • Migrations
  • Commands
• DB seeding and fixture loading
• Framework integration
  • Litestar
  • FastAPI
  • Flask