Add setup guide for new contributors (SETUP.md)

[YuhaoLin2005]

What

Create a SETUP.md (or .devcontainer.json) that helps new contributors get started in under 5 minutes.

Should include

1. Prerequisites: what you need installed (nothing for docs-only contributions; Python 3.8+ for skill testing)
2. Quick start: clone, browse, find your entry point (link to Start Here table in README)
3. How to test changes:
   • For docs: preview markdown rendering
   • For skills: install in Claude Code and test manually
4. Project layout: brief directory tree explaining what lives where
5. Where to ask questions: link to issues/discussions

Notes

• checkgrow is a methodology repo, not a code repo — setup is minimal
• Different contribution types need different setup paths
• See CONTRIBUTING.md