Adopt a Code of Conduct for community health files

[stevepiercy]

• https://github.com/collective/icalendar/community
• https://docs.github.com/en/communities/setting-up-your-project-for-healthy-contributions/adding-a-code-of-conduct-to-your-project

I have no interest in drafting this, but I support copy-pasting something that works for us, starting with whatever is already established by https://github.com/collective/.github/community (there is none).

[SashankBhamidi]

This should cross-off the checklist

[niccokunzmann]

See also: #852