docs: update README with API documentation and usage examples

[robertodauria]

Summary

• Add API endpoints table documenting all service endpoints
• Add development section with build, test, and Docker commands
• Add configuration table with all flags and defaults
• Add CLI tools documentation for create-integration
• Add usage section with integration endpoint request/response examples
• Add architecture overview with Mermaid sequence diagram

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This change is 

[bassosimone]

Wow, I like this a lot. Here's tiny request for minor improvements.

[bassosimone]

🐳