Document SSH key specification for email flow authentication

[josharian]

Author: philz42
Channel: #Sharing ssh access
Link: https://discord.com/channels/1405685085923049482/1461382624520966319/1461426868908462427

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Task

Add documentation for specifying SSH keys when authenticating through the email flow.

When using SSH to authenticate, users should be able to specify a specific key using:

ssh -f <key> -o IdentityOnly=yes

This needs to be documented for users who want to use multiple SSH keys or need to specify a particular key for authentication.

Context

Users have been asking about adding SSH keys for multi-user access scenarios. The workaround currently involves either sharing SSH keys (which shares account access) or using Tailscale SSH. Documentation on how to properly specify keys during SSH authentication would help users understand this capability.

Related discussion: Users are working with multiple team members (2+ people) who each need SSH access to a shared resource, and need guidance on key management.