Support JS-generated output like Mermaid in PDF export

[thewilli]

As stated in #682 Mermaid output is not supported in PDF exports. Nevertheless, I would really appreciate that feature and for sure others as well.

[vsch]

@thewilli, if there is a web service which can render Mermaid charts to PNG or SVG or JPEG images then it would be an easy addition as is done for Math and UML diagrams.

Otherwise, it is a problem because Mermaid renders in JavaScript and I would need a browser to do the rendering of images to be able to have them available in PDF.

I will add this to the enhancement to do list but if you come across information on how to render Mermaid charts outside the browser please let me know. It will definitely speed up the implementation if I know it is possible and have a clear idea of what I need to do. Otherwise, it will be in my To Research stack before it gets to the implementation To Do list.