[rich-iannone]

This PR addresses an issue where a Mermaid diagram with a fig-cap rendered very small, with its caption to the right of the SVG instead of underneath.

The fix makes the figure now stack things vertically (flex-direction: column; align-items: stretch), so the caption sits under the diagram. Captions are centered to match the centered diagram. Small diagrams keep their natural size, where Mermaid's inline max-width on the SVG is left alone. Uncaptioned diagrams look the same as before.

I added a new GDG site (gdtest_mermaid) with three diagrams: a wide one with a labeled caption, a small one with an unlabeled caption, and one with no caption.

Here's how figures now render with Mermaid in Great Docs:

Fixes: #361

[github-actions]

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