Add Mermaid support to our Kroki server

PlantUML chooses its own positioning for elements — it’s possible to group items, and drop hints about the overall layout of the diagram, but in cases like the linked page, where you absolutely have to insist that “this element goes on the left and this element goes on the right”, I couldn’t get it to work.

I’d like to request that we get our kroki server to support Mermaid also. I propose that the style rule should be, we use PlantUML first, unless it really can’t do the kind of diagram that you require.

The default Mermaid diagrams have a look-and-feel that’s inconsistent with our other diagrams. If possible, set the default the Mermaid font and colors to make them look more like our PlantUML diagrams.