Add verification evidence CID schema

[es3298]

Closes #236.

Summary

• Add verification-evidence.schema.json for verification evidence CIDs.
• Add verification-evidence.example.json with proof links, screenshot/IPFS proof, attestation, signature, and privacy metadata.
• Add docs/VERIFICATION_EVIDENCE.md documenting required fields, proof types, attestations/signatures, and privacy/safety expectations.
• Link the schema, example, and guide from the root README.

Validation

• Parsed verification-evidence.schema.json with PowerShell ConvertFrom-Json.
• Parsed verification-evidence.example.json with PowerShell ConvertFrom-Json.
• git diff --check

[es3298]

@Naomi-Gift this PR is ready for GrantFox review for #236. It is open and mergeable, adds the verification evidence CID schema, example evidence document, and contributor guide for proof links/signatures/screenshots/attestations. Happy to provide the GrantFox/Stellar payout destination privately after acceptance/merge.