Add Onboarding Guide for New Contributors Covering Both XcodeGen and Gradle Setup

[maugauwi-hash]

Area: Documentation
Priority: Low
Estimated Time: 1.5 hours

Description:
README.md's Setup section is accurate but dense (nine numbered iOS steps alone, including
XcodeGen, entitlements, and TLS pin configuration). A single "day one" onboarding doc distilling
just what's needed to get a debug build running (deferring release-signing/cert-pinning specifics)
would reduce new-contributor ramp-up time.

Tasks:

• Create docs/contributor-onboarding.md with a minimal "get a debug build running" path per platform
• Explicitly call out which README.md steps are release-only and can be skipped for local dev
• Link to the fuller README.md setup steps for anything beyond first-run

[belloaliyu11]

@belloaliyu11 has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

Hello Maintainer, I’d love to take ownership of this issue and submit a high-quality pull request once completed.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @belloaliyu11 to this issue.

[teagan101-cloud]

@teagan101-cloud has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

I checked the current setup before applying. I’ll write the requested contributor onboarding guide around the shortest real debug path for each platform. On iOS I’ll cover creating the Xcode directory, generating the project from project.yml, and opening the app and widget targets, while clearly separating signing, entitlements, association files, push setup, and release certificate pins. On Android I’ll document the minimal Gradle debug build and separate Firebase, app links, release signing, and certificate pins. I’ll verify each command against the repository and point readers to the full README for production setup.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @teagan101-cloud to this issue.

[rudeus112266]

@rudeus112266 has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

Kindly assign

ℹ️ Repo Maintainers: To accept this application, review their application or assign @rudeus112266 to this issue.

[drips-wave]

Congratulations, @rudeus112266! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on August 31, 2026.

🧑‍💻 @rudeus112266: 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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[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox!

@rudeus112266's PR #364 was approved and merged by @maugauwi-hash.

🏆 @rudeus112266: You earned 35 FoxPoints for this contribution! Your current tier: Explorer (153 total points). Track your full progress on GrantFox.

👏 Great work, @rudeus112266! Keep contributing to ethos-protocol.