Add doctest example for QueryState to to direct users to World::try_query for easy QueryState construction

[IronGremlin]

Objective

It's easy for users to stumble across World::query or DeferredWorld::query but then be somewhat mystified as to how one goes about constructing a QueryState without getting into a knife fight with the borrow checker and/or requiring a &mut World they may not have.

Solution

Provide users with a practical example that answers this question in a place in the docs they're likely to end up in regardless of which angle they encounter this problem from.

Testing

N/A

Showcase

N/A

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