docs: improve contributor env setup

[OkeyAmy]

Closes #20

Summary

• Provide a safe, contributor-friendly .env.example with placeholder-only values so contributors can run the app without maintainer API keys or secrets.
• Make local setup explicit and reduce support requests by documenting how to copy the template, which variables are required/optional/mock-only, and recommended mock flags for payments, email, and Stellar.

Description

• Replace live-style JWKS URL with an explicit placeholder and ensure .env.example contains the full suggested set of safe variables and mock flags only.
• Update README.md Local Development instructions to show cp .env.example .env.local, explain that .env.example is tracked while .env* is ignored, and recommend ENABLE_MOCK_PAYMENTS, ENABLE_MOCK_EMAILS, and ENABLE_MOCK_STELLAR for contributors.
• Extend CONTRIBUTING.md with environment categories that classify variables as required, optional, mock-only, or planned for Stellar, and reinforce contributor safety rules around secrets.
• Confirm no production secrets were added and .gitignore continues to exclude .env* while allowing .env.example to be tracked.

[Obiajulu-gif]

@OkeyAmy great work