Docs changes for on demand store

[tmeasday]

Part of #15871

Telescoping on #16006

What I did

Docs changes to support on-demand store.

• The format of the DocsContext changed a bit (and there were a bunch of type changes)
• Story fetching APIs can be async, so ArgsTable and Story need to have loading state

These changes were mostly pretty mechanical.

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[shilman]

Looking good!!

[shilman]

Should this be DocsContextProps<AnyFramework> as below?

[shilman]

should TFramework be AnyFramework by default? Would eliminate a lot of boilerplate

[tmeasday]

should TFramework be AnyFramework by default? Would eliminate a lot of boilerplate

I think when you are associating two types (e.g. a function that takes a StoryContext and produces a Decorator, you need the type variable so you can guarantee that the two AnyFramework are the same AnyFramework.

The analogy would be to functions that take any vs a typed variable:

// Doesn't matter what `var1` is
function isTruthy(var1: any) {
  return !!var1;
}

// Doesn't matter what `var1` and `var2` are, but they need to be same
function isEqual<T>(var1: T, var2: T) {
  return var1 === var2;
}

// This will give a type error
isEqual(1, '1');

// You could define `isEqual` in terms of `any` but it'd lose some type info
function isEqual2(var1: any, var2: any) {
  return var1 === var2;
}

// This won't give a type error, which is less useful
isEqual2(1, '1');

That's my understanding of it anyway, I could totally be wrong, my TS-fu is still pretty recent.

[tmeasday]

Oh, I totally missed what you mean here. You mean to change the various types to like DocsContextProps<TFramework extends AnyFramework = AnyFramework>.

I think that's a good idea for DocsContextProps, but doesn't really help in most other cases for the same reason I posted above. But we may as well default the type I guess.

[shilman]

Merge as part of #15871