Add a way to include the ERD as a mermaid code block in index.md #22

I personally prefer the code block option because beautiful-mermaid apparently only supports limited cardinality types, and because large diagrams are nearly unreadable because of overlapping relations.

If the erdFormat option is set to "both", it would not be clear which to use for index.md. This is why I would suggest a new option "indexErdFormat" that is "svg", "mmd", or "none".

Thank you all so much for this useful plugin!