Add example using State in docs

[BD103]

Objective

• It may be confusing whether State is a Resource or a SystemParam.
• Fixes Missing docs in prelude::State #11312.

Solution

• Add an example using State in a system in the docs, to clarify that it is a Resource.

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I basically copied the example from States and added a system beside it. I don't have a strong opinion on what the example should look like, so please comment if you have a better idea. :)

[alice-i-cecile]

Looks good. Can you add a matching one to NextState that changes the state?