Add make help target

[hugovk]

Summary

Checklist

Making a release

• Bump the version in src/pypi_attestations/__init__.py
• Add a new subheading in the CHANGELOG for the new version
• Add a link at the bottom of the CHANGELOG for the diff of the new version

Adding new public-facing APIs

• Expose the new types/functions in src/pypi_attestations/__init__.py

❯ make help
Usage: make [target]

  dev             Create development virtual environment
  doc             Generate documentation with pdoc
  edit            Open all source files in editor
  help            Display this help
  lint            Run linters (Ruff, mypy, interrogate)
  package         Build distribution packages
  reformat        Reformat source code with Ruff
  test tests      Run tests with coverage
  test-nocoverage Run tests without coverage