build: add mermaid plugin to docs (#641)

Author: dreamorosi

mkdocs.yml

@@ -62,7 +62,11 @@ markdown_extensions:
   - attr_list
   - pymdownx.emoji
   - pymdownx.inlinehilite
-  - pymdownx.superfences
+  - pymdownx.superfences:
+      custom_fences:
+        - name: mermaid
+          class: mermaid
+          format: !!python/name:pymdownx.superfences.fence_code_format
 
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