Summary

This PR adjusts the gen-build-spec tutorial for the Python and extends support for additional Python build tools, and refactors registries to make them build tool agnostic. It also improves the logic for the default build requirements and backends.

Description of changes

• Adds a comprehensive tutorial for using the Python gen-build-spec command, providing step-by-step instructions, usage examples, and best practices to help users get started effectively.
• Extends the functionality of gen-build-spec to support a wider range of build tools (e.g., flit, hatch, conda).
• Refactors registry handling logic to decouple it from specific build tools. Registries are now organized based on the PURL (Package URL) namespace, enabling a tool-agnostic approach.
• Improves the logic of default build requirements and backends for Python packages. The default setuptools is only used when no backends are declared by the project configurations.
• Updates related documentation to reflect these changes and ensure consistency.