feat: Add Antora Mermaid extension

[sbernauer]

Replaces #790
Needed by stackabletech/listener-operator#348
Example usage

[mermaid, width=100%]
....
flowchart LR
    LB{LoadBalancer supported?}
    LB -->|yes| ephermal-nodes[Use ephermal-nodes]
    LB -->|no| node-rotation{Nodes regularly rotated?}

    node-rotation -->|yes| complicated[Use stable-nodes to prefer NodePort<br> over LoadBalancers.<br> However, don't use the<br> external-stable ListenerClass at all,<br> use external-unstable instead]
    node-rotation -->|no| stable-nodes[Use stable-nodes]
....

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[NickLarsenNZ]

LGTM