Add CODEOWNERS file for automated PR reviews

Add a CODEOWNERS file to automatically assign reviewers based on file paths.

This will help ensure the right people review changes to specific parts of the codebase.

The file should include:
- Default owners for the repository
- Specific owners for critical directories (e.g., .github/workflows/)
- Owners for documentation files

This is a simple, low-risk change that improves review efficiency.