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

docs/admin/deployment/tor_pow.rst

@@ -0,0 +1,51 @@
+Tor Proof-of-Work Defense on the *Source Interface*
+===================================================
+
+The SecureDrop *Source Interface* is served as an onion service with an
+``.onion`` URL, requiring Tor Browser to access it over the Tor network.  Tor is
+sometimes targeted for denial-of-service (DoS) attacks that can `slow down the
+Tor network as a whole <https://blog.torproject.org/tor-network-ddos-attack/>`_
+as well as burden individual onion services, including SecureDrops.
+
+Tor now includes a `proof-of-work (PoW) defense
+<https://community.torproject.org/onion-services/ecosystem/technology/pow/>`_
+against denial-of-service attacks that can be turned on for individual onion
+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
+slow, it can protect your SecureDrop from being attacked specifically; and more
+onion services running with this feature helps improve the resilience of the Tor
+network.
+
+
+Enabling the proof-of-work defense
+----------------------------------
+
+Make sure you have :doc:`installed SecureDrop already
+<../installation/install>`.  Then, on the *Admin Workstation*:
+
+.. code:: sh
+
+  cd ~/Persistent/securedrop
+  ./securedrop-admin sdconfig
+
+The prompts will include::
+
+    Enable Tor's proof-of-work defense against denial-of-service attacks for the Source Interface?: yes
+
+Type <Enter> to accept the new default ``yes`` value.  When you finish the
+prompts, rerun the installation script::
+
+    ./securedrop-admin install
+
+The Tor configuration will be updated to enable the proof-of-work defense.  When
+the script finishes, confirm that you can access the Source Interface.
+
+
+Disabling the proof-of-work-defense
+-----------------------------------
+
+Follow the instructions given above for enabling the proof-of-work defense, but
+answer ``no`` at the prompt::
+
+    Enable Tor's proof-of-work defense against denial-of-service attacks for the Source Interface?: no

docs/index.rst

@@ -121,6 +121,7 @@ Get Started
    admin/deployment/onboarding_journalists
    admin/deployment/onboarding_admins
    admin/deployment/yubikey_setup
+   admin/deployment/tor_pow
    admin/deployment/https_source_interface
    admin/deployment/ssh_over_local_net
    admin/deployment/remote