[Docs] Add some documentation on how to test `preloadQuery`

[jerelmiller]

With the introduction of preloadQuery, MockedProvider is no longer useful for testing since it relies on React context to provide mocks to existing hooks. Since preloadQuery is meant to be called outside React, we no longer have this luxury.

We should provide some guidance on testing with query preloading so that users are aware of tools like MockLink to handle this.