Adopt the NumFOCUS Code of Conduct

[JuliaSprenger]

As decided in the last Neo Core Meeting, this MR replaces the current CoC with the NumFOCUS CoC. As recommended by NumFOCUS the MR does not copy the CoC, but rather links to the NumFOCUS document.

Furthermore this implies that Neo is now employing the NumFOCUS workflow for reporting CoC violations and NumFOCUS will advise Neo maintainers on how to proceed in case of a CoC violation.

The CoC also applies for events organized by the Neo Team, where the CoC will be introduced in the beginning of the event and a dedicated CoC contact person will be assigned.

Once this MR is merged I we will also announce the CoC change on the community mailing list.

[apdavison]

Hi Julia,

Thanks for starting this. I think this needs more context, I don't think we yet mention in the docs that we're a NumFocus affiliated project, so people might wonder why the CoC talks about NumFocus but not Neo. Could you update the text in doc/source/governance.rst, and perhaps also add a paragraph at the start of doc/source/contributing.rst?