Add diagram rendering

[sylhare]

Mermaid is a cool tool to render diagram from simple text instructions.

However as of now mermaid is not supported by kramdown: gettalong/kramdown#637 (that means ```mermaid is not recognized).
You could use a converter and all but I suppose for jekyll here using the div is still an alright alternative.

So we can use the following integration from a relative file and making it work this way

[sylhare]

Released in v2.3.0 😉