Add docs for environment variable setup for local client development

[Chucks1093]

Summary

New contributors have no documented reference for which environment variables are required to run the client locally, what each one does, and where to get the values. This causes friction during onboarding.

Scope

• Document all variables in .env.example with a one-line description of each
• Add a local setup section to the contributor guide covering: copying .env.example, which vars are required vs optional, and where to get testnet RPC URLs

Acceptance Criteria

• Every variable in .env.example has a description comment
• Contributor guide explains required vs optional vars
• Testnet RPC URL source is documented

ETA: 12 hours

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[T-kesh]

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[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Official Campaign campaign!

@T-kesh's PR #469 was approved and merged by @Chucks1093.

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This issue has been completed by @T-kesh as part of the Stellar Wave Program's 6th Wave 🥳

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