Initial support for mermaid.js in markdown

[ZakarFin]

Enables rendering diagrams like this:

Using snippets like this on the md-files under documentation:

 ```mermaid
sequenceDiagram
    Alice->John: Hello John, how are you?
    loop Every minute
        John-->Alice: Great!
    end

It's a bit slow to render on the client. We should try to make mermaid pre-render the SVG on build instead.

[vercel]

The latest updates on your projects. Learn more about Vercel for Git ↗︎

Name	Status	Preview	Comments	Updated (UTC)
oskari-web-nextjs	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Nov 22, 2023 10:14am

[ZakarFin]

Looks like server-side rendering is not supported on mermaid js at least yet: mermaid-js/mermaid#3650

https://github.com/hikerpig/pintora could be an option as well as it could be used to generate the SVG when building the docs.

[ZakarFin]

An example of mermaid thats rendered by Github: https://github.com/oskariorg/oskari-documentation/blob/52a6c1818c99d0022d885c512e59c4f92f904153/1%20Application%20environment/1.2%20Frontend.md