docs(core): risks of omitting useStore selector

[harry-whorlow]

From issue #1122 and a conversation with @crutchcorn, omitting the selector prop of the useStore hook will result in unnecessary re-renders.

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The interface of useStore shows it as an optional prop, and while the hook will work without the prop, it is not the recommended approach.

export declare function useStore<TState, TSelected = NoInfer<TState>>(store: Store<TState, any>, selector?: (state: NoInfer<TState>) => TSelected): TSelected;

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I've searched the Docs for references to this, and the only documentation relating to this hook was under Reactivity of basic concepts. However, it does not highlight the risks of omitting selector.

The only other references to this hook, I found in the documentation, were examples demonstrating the use of useStore. However, none of them highlighted this potential pitfall.

This PR adds clarification of useStore and examples of correct use.

[harry-whorlow]

Personally I feel it might be good to have a section for the library's hooks in the docs, with documentation on their use and explanation of their props, kind of like the api-reference but a bit more human friendly.

But, for now this is a good stop gap. 🤟