Add a Cython project template with native wheel checks

[timkpaine]

Add Python / Cython as a first-class Copier choice at python/cython, combining the Python project layout with native-wheel CI and hatch-cython.

• Compile explicit .py and .pyx example modules without C++ scaffolding or pybind11.
• Build Linux x86_64/ARM64, macOS ARM64, and Windows x86_64 wheels from clean sdists using the existing shared setup actions.
• Run installed-wheel tests through cibuildwheel and verify compiled imports in fresh environments, including wheels rebuilt from sdists.
• Add the standalone example configuration, generator/test targets, base CI coverage, and a local development guide.

Generated instance: https://github.com/python-project-templates/python-template-cython
Downstream migration: pmorissette/bt#549

Local macOS ARM64 checks pass: make gen-cython and make test-cython, six tests, distribution-content checks, 100% measured Python coverage, direct-wheel and sdist-rebuilt-wheel compiled imports. Actionlint passes for generated workflows. Cross-platform wheel builds run in the generated instance's CI.

Restore the shared README badges, Overview heading, and Copier note; the README template is identical to the pure, C++, and Rust variants. Audit follow-up restores common Makefile helpers/help output and optional wiki linting, and narrows generated-source exclusions so authored C/C++ helpers are not dropped from sdists. Standalone checks and optional wiki linting pass.