Add comprehensive documentation suit

[Ref34t]

Summary

This PR introduces the initial documentation suite for the PHP AI Client SDK, providing comprehensive guides for developers to integrate AI capabilities into their PHP applications and WordPress projects.

Documentation Added

• docs/1.intro.md - Introduction and overview of the PHP AI Client SDK
• docs/2.quick-start.md - Installation guide and basic usage examples
• docs/3.php-applications.md - Framework integration patterns for Laravel, Symfony, and other PHP frameworks
• docs/4.wordpress-integration.md - WordPress-specific features, hooks, and plugin development
• docs/5.advanced-usage.md - Advanced configuration, custom providers, and optimization techniques

Key Features Documented

• Multi-provider support - OpenAI, Anthropic, and Google AI integration
• Automatic model selection - Intelligent routing based on content type
• WordPress optimization - Deep integration with WordPress hooks and filters
• Framework compatibility - Ready-to-use patterns for popular PHP frameworks
• Production-ready patterns - Error handling, caching, and performance optimization

[Ref34t]

@felixarntz @JasonTheAdams As we are about to release v0.1.0, I prepared those docs based on WordPress Documentation Standards. Kindly Review and suggest improvments

[felixarntz]

@Ref34t Thank you for opening this PR.

It's a great idea to add more comprehensive documentation, but I think we need to plan this more carefully. The intro section looks great to me, but some of the subsequent sections may be a bit too opinionated.

Can you first open an issue instead for us to talk through this and come up with an outline for the different sections? For large PRs like this, it's usually best to discuss in an issue first.