User guide: add diagram with interactions between Ironic components

docs/user-guide/src/ironic/images/overview.plantuml

@@ -0,0 +1,37 @@
+@startuml
+person User
+package Kubernetes {
+    rectangle API {
+        artifact BareMetalHost as BMH
+        artifact Ironic as IronicCR
+    }
+    rectangle Pods {
+        component "BareMetal Operator" as BMO
+        component Ironic {
+            process Ironic as IronicSvc
+            process HTTPD
+            process DNSMasq
+        }
+        component "Ironic Standalone Operator" as IrSO
+    }
+}
+node Server {
+    node "BMC (Redfish, IPMI)" as BMC
+    process "Ironic Python Agent" as IPA
+}
+
+User --> BMH : edits
+User --> IronicCR : edits
+
+BMH <--> BMO : watches CR
+IronicCR <--> IrSO : watches CR
+
+BMO --> IronicSvc : calls
+IrSO ==0 Ironic : manages
+IronicSvc -0 HTTPD: configures
+
+IronicSvc --> BMC : calls
+HTTPD <-- BMC : downloads vmedia
+IronicSvc <--> IPA : runs commands
+HTTPD <-- IPA : downloads images
+@enduml

docs/user-guide/src/ironic/introduction.md

@@ -44,6 +44,10 @@ Metal3 provides [a way to install Ironic](../irso/introduction.md) with a
 suitable configuration. Alternatively, Bare Metal Operator can be set up to use
 an externally managed Ironic instance.
 
+### Components overview
+
+![Component overview](./images/overview.png)
+
 ### Requirements for external Ironic
 
 - HTTP basic authentication (OpenStack Identity is not supported - see [issue