Document static interface on mixin

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In [Typing the Subclass Documentation](https://lit.dev/docs/composition/mixins/#typing-the-subclass) we include how to declare the type of a subclass, but don't include handling a static interface.

It may be helpful to document handling of static properties.

Example case where a mixin adds a static property with correct typing, i.e:

```ts
// Define the interface for the mixin
export declare class MyMixinInterface {
highlight: boolean;
}

// Define static interface for mixin
interface MyMixinStaticInterface {
mixinStyles: CSSResultGroup;
}

export const MyMixin = >(superClass: T) => {
class MyMixinClass extends superClass {
@property() highlight = false;
static readonly mixinStyles = css`...`
};
// Cast return type to your mixin's interface intersected with the superClass type
return MyMixinClass as Constructor & MyMixinStaticInterface & T;
}
```

Start with the “Typing the Subclass Documentation” section linked in the issue, especially the “typing the subclass” subsection. Document how a mixin can expose and type static properties using the provided TypeScript example; done means the documentation explains the static interface alongside the existing subclass typing guidance.

Written by the indexing model from the issue text.