Add Contributor Covenant Code of Conduct v2.1

[KardelRuveyda]

What

Adds a CODE_OF_CONDUCT.md at the repository root using the standard Contributor Covenant v2.1.

Why

• The Contributor Covenant is the de-facto OSS standard (adopted by Linux, Kubernetes, TensorFlow, PyTorch, and many academic projects).
• Once merged, GitHub's Community Standards page picks it up automatically and a "Code of Conduct" link appears in the repo sidebar.
• Sets clear, friendly expectations for contributors and gives maintainers a documented enforcement path if something goes wrong — without committing to a heavy moderation workload.

Choices made

• v2.1 (the current stable Contributor Covenant version) — verbatim text where the license requires it, so it is instantly recognisable to contributors who have seen it elsewhere.
• Reporting contact: points to the corresponding author already listed in CITATION.cff / the publication. No new mailing list or alias is required.

Notes for the maintainer

• Docs-only change; no code or build touched.
• Feel free to swap the contact pointer for a dedicated conduct@… alias later if you ever set one up — only one line needs to change.