Implement PROPOSAL 165: code of conduct update and email

[wlandau]

This pull request implements r-multiverse/help#165:

1. Updates the code of conduct to Contributor Covenant v3.0.
2. Uses conduct@r-multiverse.org as the point of contact for reports.

I do realize we have no permanent CoC or committee yet. Currently the admins have access to this email and get forwarded copies of reports, but this might not be appropriate or desired. @shikokuchuo, @maelle, and @jeroen, please let me know if you are willing to stay on or if you would like me to remove you from the inbox.

I have a strong preference for using conduct@r-multiverse.org as the email, even if I am the only one with access until we get a permanent CoC and committee.

Remarks:

• PROPOSAL: update to Contributor Covenant v3.0 help#165 was posted on September 13 2025, so our governance gives us until October 13 2025 to vote on the proposal.
• This PR supersedes PROPOSAL: code of conduct committee email #50 and List code of conduct committee #51.

[wlandau]

Just changed the draft code of conduct based the updated roles. @maelle, does it look okay to you?

[maelle]

Yes, thank you 🙏

[wlandau]

It's October 22. Time to merge.