document Tor proof-of-work defense

[cfm]

Describe the change

What documentation should we offer for the Tor proof-of-work defense implemented in freedomofpress/securedrop#7175?

• Just how to enable it
• How to monitor it (non-invasively) via onionprobe
• More? something else?

How will this impact SecureDrop users?

Additional context

[cfm]

@zenmonkeykstop and @nathandyer, let me know what documentation you think will be most useful here, and I'll draft it.

[nathandyer]

My instinct here is that this is the sort of feature that requires minimal documentation, as it doesn't really have many user-facing implications. I don't expect that active monitoring will be something that admins will need to do, and I think even mentioning onionprobe might be a bit far since it requires the extra package install in Tails (and is broken in bookworm/Tails6.x).

I think documenting this as part of the ./securedrop-admin sdconfig workflow is likely sufficient, noting that the option exists, and briefly explaining why an admin may or may not want to enable it (similar to how we handle the SSH-over-Tor option).

[cfm]

Thanks, @nathandyer. This is ready for review in #569.