Expand README with usage, attributes and architecture overview

[piotrooo]

Summary

• Rewrite README to clearly state what the library does (generate OpenAPI 3.0.1 from Ouzo RouteRules).
• Add Requirements, Installation, three-step Usage (provider → customizer → generate), #[Hidden]/#[Schema] examples, polymorphism note, and a brief architecture sketch (PathsService/ComponentsService/OpenApiCustomizersRepository).
• Replace absolute GitHub links with repo-relative ones.

Test plan

• Render README on GitHub and verify all section anchors and code blocks display correctly.
• Confirm relative links (src/OpenApi/RouteRulesProvider.php, src/OpenApi/Customizer/OpenApiCustomizer.php) resolve on the merged commit.

🤖 Generated with Claude Code