[DOCS] Write environment variable configuration guide

[dev-fatima-24]

Priority: high

Description

The README lists environment variables in a table but lacks format rules, examples, and guidance on obtaining values (e.g., how to get a Soroban RPC URL). A dedicated config guide is needed.

Acceptance Criteria

• Every variable has a description, type, example value, and whether it is required
• Format rules are documented (e.g., key prefixes, URL format)
• Instructions for obtaining each value (Horizon URL, contract ID, etc.) are included
• Guide is stored in docs/configuration.md
• .env.example is kept in sync with the guide

[Markodiba6399]

@Markodiba6399 has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

I would like to apply for this issue

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[drips-wave]

Congratulations, @Markodiba6399! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 2, 2026.

🧑‍💻 @Markodiba6399: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

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