[Beta] Initial API doc for Suspense

[acdlite]

This doesn't include everything but I think it's good enough for a start.

I didn't add any interactive examples yet because I want to use the use API, so I'll wait until that's documented.

[github-actions]

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[gaearon]

If we're comfortable adding a dependency on launch, I feel like you could make the sandboxes use the experimental build and have them use it. WDYT?

[gaearon]

I think the "code step" usage here is a bit more distracting than helpful. I'd prefer if we keep it limited and instead of highlighting pieces of JSX in code, we use it for terms like "content" and "loading indicator" and try to use at most three (ideally two) colors at the time for any given explanation. We also don't have to highlight all matches. E.g. I think it would actually be better to remove highlights here but add them in the sentence above the snippet for "its children" and "fallback prop" instead.

Ditto for sections below.

[gaearon]

let's tweak the highlight thing.

ideally we need some example. e.g. at least lazy(). i think it should work in the sandbox?

use() with experimental build could also be nice. you can override which react build is used by specifying a package.json file in the sandbox. you can search markdown for examples of package.json file, we use this for adding Immer to some pages

[gaearon]

any appetite for adding the sandbox with lazy? i was thinking maybe add a large markdown library as a dep of a component and trigger it on click. might be enough to show the lazy loading if codesandbox actually supports dynamic imports (i think it does?)