Add diagram to explain protobuf message structure

docs/using-raw-protobuf.md

@@ -0,0 +1,32 @@
+# Using the Protobuf protocol directly
+
+## Example diagram
+
+``` mermaid
+classDiagram
+  Person <|-- Student
+  Person <|-- Professor
+  Person : +String name
+  Person : +String phoneNumber
+  Person : +String emailAddress
+  Person: +purchaseParkingPass()
+  Address "1" <-- "0..1" Person:lives at
+  class Student{
+    +int studentNumber
+    +int averageMark
+    +isEligibleToEnrol()
+    +getSeminarsTaken()
+  }
+  class Professor{
+    +int salary
+  }
+  class Address{
+    +String street
+    +String city
+    +String state
+    +int postalCode
+    +String country
+    -validate()
+    +outputAsLabel()
+  }
+```

mkdocs.yml

@@ -41,7 +41,12 @@ markdown_extensions:
   - pymdownx.magiclink
   - pymdownx.mark
   - pymdownx.smartsymbols
-  - pymdownx.superfences
+  - pymdownx.superfences:
+      custom_fences:
+        - name: mermaid
+          class: mermaid
+          format: !!python/name:pymdownx.superfences.fence_code_format
+
   - pymdownx.tilde
   - pymdownx.snippets
   - pymdownx.tabbed:
@@ -97,6 +102,7 @@ nav:
   - "Visualizing with Foxglove": "foxglove-bridge.md"
   - "Forwarding positioning to NMEA": "nmea-publisher.md"
   - "Updating from v1 to v2": "migrating-to-v2.md"
+  - "Using the Protobuf protocol directly": "using-raw-protobuf.md"
   - "HTTP API": "http-api.html"
   - Reference:
       - blueye.sdk.battery: "reference/blueye/sdk/battery.md"