Integration of kroki

[erikhofer]

https://kroki.io/ is a server for rendering diagrams from text. Requests to external services are not optimal, but it would support a lot of diagram formats with minimal effort.

This should be opt-in and configurable, so that a private self-hosted server can be used.

This would also play together nicely with the docToolchain integration if docToolchain/docToolchain#531 gets implemented.

[erikhofer]

On the one hand, we should support simple code blocks, just like we already do with mermaid. Example:

```plantuml
Bob->Alice : Hello!
```

On the other hand, we need the ability to do some configuration, e.g. specifying the view of a structurizr diagram (example).

References we should look at:

• https://github.com/Mogztter/asciidoctor-kroki
• https://docs.gitlab.com/ee/administration/integration/kroki.html
• https://pypi.org/project/mkdocs-kroki-plugin/ (Usage of a kroki prefix, less common)