Add architecture diagram

src/SUMMARY.md

@@ -1,5 +1,7 @@
 # Summary
 
+- [Architecture]()
+  - [Overview](./architecture/overview.md)
 - [Jenkins Pipeline]()
   - [Pipeline setup](./jenkins-pipeline/setup.md)
   - [Pipeline template](./jenkins-pipeline/template.md)

src/architecture/overview.md

@@ -0,0 +1,16 @@
+# Overview
+
+## TL;DR
+
+![Overview](../public/diag-high-level-overview.png)
+
+## Hub API
+
+<style>
+    .content main {
+        margin: 0;
+    }
+    img {
+        min-width: 75vw;
+    }
+</style>