Add model_to_mermaid to docs (#7868)

Author: williambdean

docs/source/api/model/core.rst

@@ -29,5 +29,6 @@ Graph visualization
 .. autosummary::
    :toctree: generated/
 
-   model_to_networkx
    model_to_graphviz
+   model_to_mermaid
+   model_to_networkx