docs: add contributor onboarding guide (XcodeGen + Gradle)

[rudeus112266]

Summary

Adds docs/contributor-onboarding.md, a minimal "get a debug build running" path for both iOS (XcodeGen) and Android (Gradle), distilled from README.md's full Setup section.

Context

README.md's Setup section is accurate but dense — nine numbered iOS steps alone, mixing local-dev essentials with release-signing and certificate-pinning specifics. New contributors only need a small subset of that to get a debug build running. This doc:

• Gives the minimal per-platform steps to a working debug build
• Explicitly calls out which README.md steps are release-only and safe to skip for local dev (iOS steps 5-8; Android steps 2, 3, 5)
• Links back to README.md for the full setup/testing steps once a contributor is ready to go beyond first-run

Testing

Documentation-only change. Cross-checked each "skip for local dev" callout against the current README.md Setup section and the CI pin-check scripts to confirm debug builds are genuinely exempt.

Closes #304

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