Should prevent misunderstandings like #16092

This PR clarifies in the testing documentation that testing examples should not use $state for arrays that are mutated. It also adds a separate untrack section in the $effect documentation to warn users about updating $state values while depending on those same values within an effect.

This should help clarify common misunderstandings users have when dealing with $state versus regular variables.

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Tests and linting

• Run the tests with pnpm test and lint the project with pnpm lint