Add CODEOWNERS file

[nethermind-oss-compliance]

📋 Add CODEOWNERS File

This PR adds a CODEOWNERS file template to help manage code ownership and review requirements.

🎯 What is CODEOWNERS?

The CODEOWNERS file defines who is responsible for code in this repository. When someone opens a pull request, the defined code owners are automatically requested for review.

✏️ How to customize this file:

1. For team ownership: Use @NethermindEth/team-name
2. For individual ownership: Use @username
3. For specific paths: Add patterns like docs/ @NethermindEth/docs-team

📖 Examples:

# Global owners (will be requested for all PRs)
* @NethermindEth/core-team

# Specific directory owners
/docs/ @NethermindEth/docs-team
/src/ @NethermindEth/development-team
*.md @NethermindEth/docs-team

# Individual file owners
package.json @username
README.md @NethermindEth/docs-team @username

🔗 Learn more:

• GitHub CODEOWNERS documentation

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Please review and update the CODEOWNERS file according to your team's needs before merging! 🚀