build(gitlab): add template for merge request

Summary

Add a gitlab merge request template in .gitlab/merge_request_templates/default.md.

🚨 Breaking changes

None.

⚙️ Implementation

None.

🧪 How to test this MR

Look at this MR.

🏞️ Screenshots

Show your work !

✔️ Checklist

• I have updated CONTRIBUTORS.md with my name if it is my first contribution
• I have updated or created unit tests when applicable
• I have updated or created documentation for the changes