Cellar: update SDK examples to v3 + policy/formatting fixes

[BlackYoup]

📝 What does this PR do?

Updates Cellar documentation to use modern AWS SDK v3 (Node.js), boto3 (Python), and latest AWS SDK v2 (Java). Adds checksum validation configuration workarounds for SDK compatibility with current infrastructure. Fixes read-only policy resource
definition and code formatting.

🔗 Related Issue (if applicable)

• N/A

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🧪 Type of Change

• ⚠️ Bug fix
• 📅 Changelog update
• 📚 Documentation update
• ✨ New content/feature
• 🔧 Technical/maintenance

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✅ Quick Checklist

• I have read the contributing guidelines
• The content is accurate and links work
• The site builds without errors

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👥 Reviewers

@CleverCloud/reviewers

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📋 For major changes (click to expand)

Additional testing performed

• Updated SDK examples tested with latest versions (AWS SDK v3.729.0+, Java SDK 2.40.1, boto3)
• Verified checksum validation workarounds resolve compatibility issues
• Validated bucket policy JSON syntax

Breaking changes

• Old boto examples replaced with boto3 (Python users need migration)
• AWS SDK v2 replaced with v3 for Node.js (users need dependency updates)
• No actual breaking changes to Cellar service itself

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