chore(docs): add useStaticQuery to docs for mocking Gatsby

[entorenee]

Description

Adds useStaticQuery to the docs for mocking out Gatsby related to unit testing.

[DSchau]

This is great--thanks!

Thinking it's getting pretty high-time to make this a package, similar to what we do with cypress-gatsby.

It'd be a suite of utils and mocks that make working with core Gatsby APIs even easier from a unit testing perspective.

I'll open an issue on this tomorrow with some of my thoughts!

[entorenee]

That sounds really intriguing and curious to see the discussion on it. Experimenting with some higher level integration tests which may have multiple sub components that are using StaticQuery. I also wonder if that may be a place of either increased documentation and/or related to this package idea.