Initial Float docs for hoistable/semantic elements

[davidmccabe]

@gnoff thanks for reviewing #6459, here's the second half. Please review especially for accuracy.

The thing I'm most unclear on is the relationship between the renderable elements and the imperative functions. If you call the functions like preinit (which are described as "hints" in the RFC), do you also have to render the elements to make sure? Or just one or the other?

https://react-mpvnsvyqg-fbopensource.vercel.app/reference/react-dom/components#resource-and-metadata-components

[github-actions]

Size changes

📦 Next.js Bundle Analysis for react-dev

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Three Pages Changed Size

The following pages changed size from the code in this PR compared to its base branch:

Page	Size (compressed)	First Load
/404	78.03 KB (🟡 +693 B)	181.85 KB
/500	78.03 KB (🟡 +693 B)	181.84 KB
/[[...markdownPath]]	79.67 KB (🟡 +737 B)	183.48 KB

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