Add Vitest cleanup hint to react-testing-library setup docs

[ezzatron]

This PR adds a heading to the setup guide for react-testing-library that explains how to get auto-cleanup working when using Vitest. The wording was inspired by a similar heading in the vue-testing-library setup guide: https://testing-library.com/docs/vue-testing-library/setup/#vitest

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[ezzatron]

I had a crack at documenting how to get it working without enabling globals. Let me know what you think.

[ezzatron]

Just following up to ask if there's anything else needed to progress this PR?

[MatanBobi]

@all-contributors please add @ezzatron for docs :)

[allcontributors[bot]]

@MatanBobi

I've put up a pull request to add @ezzatron! :tada: