add figures from ignn23a course notes

fig/course_notes/2023-04-01-zjost-ignn23a/message-passing-01-intuition.org

+#+begin_src mermaid :file message-passing-01-intuition.png
+%% mermaid graph showing basic message passing intution
+%% for graph neural networks.
+graph LR
+%% subgraphs
+    subgraph one[message]
+    a1((A))-- fa:fa-envelope ---c1((C)):::orange80
+    b1((B))-- fa:fa-envelope ---c1
+    c1-- fa:fa-envelope ---d1((D))
+    end
+    subgraph two[aggregate]
+    c2((C)):::orange80-. All messages .->c2'((C)):::orange50
+    end
+    subgraph three[update]
+    a3((A))---c3'((C)):::orange50
+    b3((B))---c3'
+    c3'---d3((D))
+    end
+%% inter-subgraph
+    one ==> two
+    two ==> three
+%% styling
+   classDef orange80 fill:#ffcc99
+   classDef orange50 fill:#ff8000
+
+%% references
+%% https://mermaid.js.org/syntax/flowchart.html
+%% https://www.w3schools.com/colors/colors_picker.asp
+#+end_src
+
+#+RESULTS:
+[[file:message-passing-01-intuition.png]]