Add support for Mermaid diagrams

[eriksjolund]

GitHub renders diagrams from Mermaid code that is included in Markdown (see GitHub docs).
For instance these lines

https://github.com/containers/podman/blob/287a4193e70301a63908ca8529eb10c39265ee24/docs/tutorials/socket_activation.md?plain=1#L20-L24

are rendered as

stateDiagram-v2
    [*] --> systemd: first client connects
    systemd --> podman: socket inherited via fork/exec

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when viewed in the Podman socket activation tutorial located at GitHub
https://github.com/containers/podman/blob/main/docs/tutorials/socket_activation.md

The same Mermaid diagram is not rendered at the web page
https://podman.io/docs/tutorials/socket_activation
instead the Mermaid code is displayed.

I found a web page in the Docusaurus docs about Mermaid support
https://docusaurus.io/docs/markdown-features/diagrams