docs(guide): add KONTRA-GUIDE.md (editing/media/admin/ops); update archify+mermaid to git-files media (drop Garage)

docs/kontra-architecture.mmd

@@ -15,12 +15,12 @@ flowchart LR
     B --> MD[(Markdown render<br/>front-matter + shortcodes)]
   end
 
-  subgraph media["Media (Garage S3 on .13)"]
-    S3[(Garage S3<br/>kontra-day bucket)]
+  subgraph media["Media (git files)"]
+    F[(Images beside articles<br/>in the content repo)]
   end
 
   G -->|"SSH git"| P
-  B -->|"shortcode: media"| S3
+  B -->|"{{media:slug/file}}"| F
   B -->|"emits HTML"| CB[Caddy master<br/>.35 :80/:443]
   CB -->|"reverse_proxy :8600"| B