Add support for gitbook 3

[octera]

In gitbook 3 the api changed. The mermaid.js in no more included in index.html

[genzj]

@octera I created a pr trying to fix this. You can try the fix before merging by add the mermaid-2 plugin like:

    "plugins": [
        "mermaid-2@git+https://github.com/genzj/gitbook-plugin-mermaid-2.git#gitbook-3"
    ],