Add notes about email addresses and groups

This adds a new section to our working conventions, describing how we use HF email addresses. There are a few policy prescriptions here, but nothing that would seem to require an official vote. I would love feedback on this, regardless.

approved this merge request

approved this merge request

added 10 commits

• c6501cf8...8f69d6d6 - 8 commits from branch main
• e85b9065 - Add notes about email addresses and groups
• 884b0654 - Clarify who can create users and groups

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merged

mentioned in commit 742b6721