kroki.io support

[ethereal-engineer]

Is your feature request related to a problem? Please describe.

Nope. Describe the solution you'd like

Being able to use my own locally hosted server to render kroki.io diagrams (which contain PlantUML, Mermaid, and a slew of others) using a syntax like:

```kroki
<content here>
```

Describe alternatives you've considered

I could just use the PlantUML feature, but it is only a subset of what kroki.io offers, as a meta-renderer, and with PlantUML already supported, inclusion of such a powerful tool should not be a huge development effort. Additional context

Coming from a WikiJS long-time user background. Check their list of supported Markdown-rendering features.

Keep up the fantastic work. This is seriously the BEST (most intuitve just-works natural keyboard shortcuts use) webapp markdown editor available (for self-hosting). I've tried them all!

[DerMolly]

To clarify: this issues is specifically for the inclusion of kroki.io and not plantuml in general, correct? The later is already part of the current plans for HedgeDoc 2.0. And I want to prevent a "cleanup the issues" session in 4 month or so mistakenly closing this issue…

[mrdrogdrog]

PlantUML with custom render endpoint URL is already included. Please change this issue to only request kroki.

[ethereal-engineer]

Done!