Preview directly the final website

**Is your feature request related to a problem? Please describe.**
The default preview is usually very different from the actual website, making it hard to visualize the final result. While it is possible to customize it, it basically requires me to code my website twice, once for the actual website and one for the preview. Moreover, I don't think it is possible to actually extract the content from other .md files, for instance to print a menu or a list previewing other pages.

**Describe the solution you'd like**

Of course, I understand this is impossible to get easily for some framework like hugo without relying on WASM. But for js CMS based on react/vuejs/svelte, it might be possible to have a better solution. For instance TinaCms provides a react hook that can be used while building the website that will pick the new content when executed from the preview tab.

https://tina.io/docs/features/data-fetching/
https://tina.io/docs/contextual-editing/overview/

**Describe alternatives you've considered**

Manualy coding a new style for the prev, see drawbacks in first section.

**Additional context**

See also https://github.com/decaporg/decap-cms/discussions/7178