Document Per-Environment Configuration Setup

### What is it?

Create documentation at `/docs/environments.md` that explains how to configure and deploy this template in a Dev/Test/Prod environment setup, including environment-specific variables, state handling, and promotion best practices.

### Definition of Done

- Written guide or markdown in `docs/environments.md`
- Includes at least one usage example per environment
- Mentions tooling or workflow assumptions (e.g., GitHub Actions, Terraform Cloud, etc.)

### Value Provided

Helps teams adopt the pattern correctly and consistently across environments.

Part of #22