add test set-up figure from main repo's deployment readme

Signed-off-by: cinereal <cinereal@riseup.net>

architecture-docs/architecture.md

@@ -202,6 +202,10 @@ Tests created so far cover:
 - builds for the (virtual) machines making up our internal infrastructure
 - ensuring any checks exposed by our Nix flake have been exposed in CI
 
+An illustration demonstrating the set-up of our deployment tests covering our applications is shown below:
+
+![](./test-setup.png){ width=100% }
+
 The latest info on these may be found at:
 
 - CI/CD runner set-up: <https://git.fediversity.eu/Fediversity/Fediversity/src/branch/main/machines/dev/forgejo-ci>

architecture-docs/test-setup.mmd

@@ -0,0 +1,38 @@
+flowchart LR
+
+  classDef invisible fill:none,stroke:none
+
+  subgraph left [" "]
+    direction TB
+
+    deployer["deployer<br><font size='1'>has store paths<br>runs nixops4</font>"]
+    client["client<br><font size='1'>Selenium scripts</font>"]
+  end
+
+  subgraph middle [" "]
+    subgraph target_machines["target machines"]
+      direction TB
+
+      garage
+      mastodon
+      peertube
+      pixelfed
+    end
+  end
+
+  subgraph right [" "]
+    direction TB
+
+    acme["acme<br><font size='1'>runs Pebble</font>"]
+  end
+
+  left ~~~ middle ~~~ right
+  class left,middle,right invisible
+
+  deployer -->|deploys| target_machines
+
+  client -->|tests| mastodon
+  client -->|tests| peertube
+  client -->|tests| pixelfed
+
+  target_machines -->|get certs| acme