docs: Render mermaid graphs (#568)

Add the plugin as a dependency as per [installation guide](https://github.com/fralau/mkdocs-mermaid2-plugin#installation).
`poetry add mkdocs-mermaid2-plugin --group docs`

Add to the `mkdocs.yml` via [plugins
syntax](https://www.mkdocs.org/dev-guide/plugins/)

This on it's own is sufficient to get the diagrams to render.

Resolves: #567

mkdocs.yml

@@ -56,7 +56,11 @@ markdown_extensions:
       emoji_generator: !!python/name:material.extensions.emoji.to_svg
   - pymdownx.inlinehilite
   - pymdownx.snippets
-  - pymdownx.superfences
+  - pymdownx.superfences:
+      custom_fences:
+        - name: mermaid
+          class: mermaid
+          format: !!python/name:pymdownx.superfences.fence_code_format
   - tables
   - attr_list
   - md_in_html