Strong support for diagrams

[damithc]

As project documentation is one of our target use cases, we should be able to say we have the best support for diagramming.

• Strengthen support for PlantUML
• Possibly, add support for other type of diagramming tools (?)

[crphang]

Some more diagram support with libraries like https://github.com/mermaid-js/mermaid could help.

Further abstraction the work already done for inline PUML: #968 can allow integration with these diagrams to be done more easily.

[ang-zeyu]

I'm almost done with the core support for this as mentioned in #968
( PR #1047 )

but there's still a design decision to be made for the markdown-it patch

Specifically, the standard behaviour of markdown-it is such that something like this dosen't work:

<div>
<script>
alert("hello!")

alert("world")
</script>
</div>

The script ( special ) tags will not work at all, unless

• you leave an empty line between the first opening <div> and the first opening <script> tag,
• or remove the empty line between the two alerts.

This actually adheres to commonmark spec, hence, do we want to follow this behaviour?

The best user behaviour for any custom tag I have in mind would of course instead be that the special tag works even without a <htmltag> directly on top of it.
( it would work if there is only the empty line between the alerts, but not the <div> before it )

The full list of html blocks ( the <div>s ) that would cause this ( point 6 here )

[damithc]

Adding support for this is useful/feasible? https://visjs.org/

[damithc]

Let's continue to try to integrate Mermaid, as it can bring in a significant value to our diagrams support.