Ontology: Add a code of conduct to the repo

[Ludee]

Description of the issue

As discussed in the dev meetings we want to implement a written code of conduct to improve discussions and collaborative work.

Workflow checklist

• [ ] Decide for a template
• [ ] Adapt if needed
• [ ] Include in CONTRIBUTING.md

Existing COC

• https://www.contributor-covenant.org/
• https://www.python.org/psf/conduct/
• https://docs.github.com/en/github/building-a-strong-community/adding-a-code-of-conduct-to-your-project#adding-a-code-of-conduct-using-a-template
• https://docs.github.com/en/github/site-policy/github-community-forum-code-of-conduct
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[stap-m]

We refer to the code of conduct in the planned paper, but it's not yet there... We should either remove it from the text or elaborate a 1st version before submitting...