Document "view-helpers" in view customization guide.

[JasonStoltz]

Currently, we recommend that users copy the default view and create a new view using the default view as a base. However, certain imports, like view helpers are imported relatively. A user creating a custom view would need to figure out how to make those absolute imports, so "@elastic/react-search-ui-views/view-helpers" ... which is not so intuitive. I think users are already doing this though, so we should just make it a concrete way of using our library and document it properly.

[botelastic]

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