[docs] Document OverflowList item key requirement

[juliosuas]

Summary

Documents the visibleItemRenderer key requirement on the OverflowList docs page.

Fixes #7746.

Testing

• pnpm exec prettier --check packages/core/src/components/overflow-list/overflow-list.mdx
• git diff --check

Notes

I also tried pnpm docs-data, but it failed while generating docs for datetime2 because @blueprintjs/datetime could not be resolved in this local environment, before any issue with this MDX change was reported.

[palantirtech]

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[docs] Document OverflowList item key requirement

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• Feature (Adding new functionality)
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• Break (Creating a new major version by breaking public APIs)
• Deprecation (Removing functionality in a non-breaking way)
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💡 Improvements

• Document OverflowList item key requirements (#8164)