ci: Add codeowners

[ko3n1g]

What does this PR do ?

We use a GH team to manage (and hide) the team of code-owners: https://github.com/orgs/NVIDIA/teams/nemo-aligner-reviewers

This is important: Whereas in GitLab the team members are transparent, here in GitHub is it not. I think this is also desired so it fits our requirements quite well.

I actually don't know if you want CODEOWNERS, it just randomly appeared me that we could do this easily. Feel free to close this PR if you want to keep it more ad-hoc.

Changelog

• Please update the CHANGELOG.md under next version with high level changes in this PR.

Usage

• You can potentially add a usage example below

# Add a code snippet demonstrating how to use this 

Before your PR is "Ready for review"

Pre checks:

• [ ] Make sure you read and followed Contributor guidelines
• [ ] Did you write any new necessary tests?
• [ ] Did you add or update any necessary documentation? Make sure to also update the NeMo Framework User Guide which contains the tutorials

Checklist when contributing a new algorithm

• [ ] Does the trainer resume and restore model state all states?
• [ ] Does the trainer support all parallelism techniques(PP, TP, DP)?
• [ ] Does the trainer support max_steps=-1 and validation?
• [ ] Does the trainer only call APIs defined in alignable_interface.py?
• [ ] Does the trainer have proper logging?

Additional Information

• Related to # (issue)