docs: add precipitation reviewer path

[stacknil]

Summary

• Add a lightweight reviewer path for the compact precipitation diagnostics mini-lab.
• Organize the review flow into 30-second orientation, 5-minute workflow review, 15-minute reproducibility review, and boundary checks.
• Reinforce that this is a supporting spatiotemporal diagnostics module, not the repository flagship release surface.

Validation

• Documentation-only change.
• New file uses relative links only and contains no local paths, raw dataset references, identifiers, or secrets.