Refactor to use OpenQASM3 AST instead of string concatenation; update README and docs with usage examples

[gstenzel]

Currently, OpenQASM3 code is generated using naive string concatenation, which is error-prone and difficult to maintain. Refactor the codebase to construct OpenQASM3 output using its Abstract Syntax Tree (AST) for improved reliability and maintainability.

Additionally, update the README and relevant documentation to:

• Explain the new approach leveraging the OpenQASM3 AST.
• Provide clear examples demonstrating how to generate OpenQASM3 code using the AST.
• Illustrate how to call and use these features in practical scenarios.

This update will make the codebase more robust and the documentation more helpful for users adopting these features.

[gstenzel]

Probably, the CONTRIBUTING.md should also add a section about using uv and poe.