docs: optimize the display effect of mermaid image

[yuluo-yx]

Such as: https://vllm-semantic-router.com/docs/overview/architecture/system-architecture#high-level-architecture-overview

When I want to look at the mermaid image carefully, the font is very small, which affects the reading experience. When I want to zoom in to see it more clearly, I can only zoom in the browser page. emmm~

When clicking on the mermaid image, a modal box is displayed to zoom in. This is the method I have thought of so far.

@rootfs @Xunzhuo WDYT?

[Xunzhuo]

looks good @yuluo-yx