Allow use of external Mermaid (and possibly other diagram types) via Kroki

Release notes

Provide support for use of external Mermaid rendering via Kroki.

Problem to solve

GitLab supports the rendering of diagrams and flowcharts via Mermaid, PlantUML and Kroki.

When the Kroki integration is enabled, the configured Kroki endpoint is used for rendering a selection of supported diagrams and charts. While this includes an extensive list, including PlantUML, it does not include Mermaid, and all Mermaid rendering is directed and limited to the version bundled with GitLab.

Proposal

We should either add Mermaid to the list of supported Kroki diagram types, or provide a method to use any diagram type supported by the Kroki server, for example being able to specify:

```kroki:mermaid
graph TD;
  A-->B;
  A-->C;
  B-->D;
  C-->D;
```

or

```kroki:wireviz
connectors:
  X1:
    type: D-Sub
    subtype: female
    pinlabels: [DCD, RX, TX, DTR, GND, DSR, RTS, CTS, RI]
...
...
```

This would also reduce the overhead of maintaining the list of supported diagram types and provide maximum flexibility, allowing the use of new diagram types as they become available to Kroki.