docs(CoC): add CODE_OF_CONDUCT.md

[gamebits]

All Submissions

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request

Adds a Code of Conduct file (CODE_OF_CONDUCT.md) to the repo, adapted from the Contributor Covenant.

Closes #2861.

How to test the changes in this Pull Request

After merging the branch, the CoC will be available for reading at

https://github.com/automattic/newspack-plugin?tab=coc-ov-file#readme

Other information

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?