docs: document Tor proof-of-work defense via securedrop-admin sdconfig

[cfm]

Status

Ready for review

Description of Changes

Closes #568 by:

1. explaining Tor's proof-of-work defense; and
2. giving instructions on how to enable and disable it.

Testing

• Visual review.

• Is this a fair summary of our decision to enable this feature in sdconfig by default going forward?

  securedrop-docs/docs/admin/deployment/tor_pow.rst

  Lines 13 to 15 in 1b9209e

  	services. As of SecureDrop 2.9.0, new SecureDrops have this feature enabled by
  	default, and we encourage all SecureDrop administrators to turn it on for their
  	instances. While this measure can't speed up the Tor network as a whole if it's

Release

This should be called out prominently in the v2.9.0 release notes.

Checklist (Optional)

• Doc linting (make docs-lint) passed locally
• Doc link linting (make docs-linkcheck) passed
• You have previewed (make docs) docs at http://localhost:8000

[nathandyer]

This mostly LGTM, I think the explanation here makes sense and is straightforward as to why an admin might want to consider enabling the option. I left one comment regarding the workflow in regards to whether or not SecureDrop has to already be installed before this option can be enabled.

I'm also wondering if perhaps we should link back to this from this section in the docs: https://docs.securedrop.org/en/stable/admin/installation/install.html#configure-the-installation (We don't explicitly mention other options here, such as SSH-over-Tor, so I'm happy either way we decide here)

[cfm]

Thanks, @nathandyer! I've replied to your review feedback inline.

I don't feel strongly either way about linking to these instructions from #569 (comment). We want this to be a transparent improvement to SecureDrop's Tor performance, rather than something administrators should have to think about. (Though we should absolutely flag it in the release notes as a heads-up.)

[nathandyer]

With the most recent commit, everything here LGTM.