add support for mermaid.js

[rsteph-de]

I want to support mermaid.js diagramms in our documentation ...

Mermaid lets you create diagrams and visualizations using text and code.
It is a JavaScript based diagramming and charting tool that renders Markdown-inspired text definitions to create and modify diagrams dynamically.

Syntax: https://mermaid.js.org/intro/syntax-reference.html

The integration is easy,

• see: https://gohugo.io/content-management/diagrams/#mermaid-diagrams
  -copy the Javascript files to our page, to avoid CDN-Server

I'll provide a shortcode for integration. It can be used as follows:

{{< mcr-figure label="Klassendiagramm" caption="MCRContent" >}}
{{< mcr-mermaid >}}
classDiagram
class MCRContent {
  <<abstract>>
  #String name
  + InputStream getInputStream()*
  + Document asXML()
  + MCRContent ensureXML()
  + byte[] asByteArray()
}
class MCRXMLContent {
<<abstract>>
+ MCRXMLContent MCRXMLContent()
}
MCRContent <|-- MCRXMLContent
class MCRJDOMContent {
+ MCRJDOMContent(Document)
+ MCRJDOMContent(Element)
}
MCRXMLContent <|-- MCRJDOMContent
{{< /mcr-mermaid >}}
{{< /mcr-figure >}}