docs: write testnet onboarding guide

[misrasamuelisiguzor-oss]

Description

Write a comprehensive testnet onboarding guide that walks a new developer through the full end-to-end flow on Stellar testnet: funding a testnet account with Friendbot, deploying the COMEBACKHERE contracts, creating an invoice, paying it with USDC, and executing a settlement.

Requirements and context

• Cover Friendbot funding and USDC test-asset setup
• Include step-by-step contract deployment using scripts/deploy_testnet.sh and .env.testnet
• Walk through creating and paying an invoice via the backend API or CLI
• Show how to verify settlement execution and check artifacts/addresses.json
• Follow existing project conventions

Suggested execution

1. Fork the repo and create a branch

   git checkout -b docs/testnet-onboarding-guide
   

2. Implement changes
3. Test and commit

Example commit message

docs: add testnet onboarding guide for account setup through settlement

Guidelines

• Assignment required before starting
• PR description must include: Closes #[issue_id]
• Keep PR description professional and detailed

[vstudio79]

@vstudio79 has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hi can I be assigned to this issue?

ℹ️ Repo Maintainers: To accept this application, review their application or assign @vstudio79 to this issue.

[jhayniffy]

@jhayniffy has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

hi maintainer can i please work on this

ℹ️ Repo Maintainers: To accept this application, review their application or assign @jhayniffy to this issue.

[drips-wave]

Congratulations, @jhayniffy! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

🧑‍💻 @jhayniffy: 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.

Warning

When opening a PR, please link it to this issue to ensure it gets tracked accurately. Points are awarded when this issue is marked as completed by the maintainer.

🤠 Repo maintainers: Please keep an eye on the contributor's progress and review their work before the due date. You can manage this issue, including adjusting its complexity and points, here.

🌊 Happy Wave 🌊