poc(composition): top-level composition

[Haroenv]

• no nested index support
• simpler than alternative
• facets should work out of the box (maybe code in SearchResponse needs to be conditional too?)

alternative: #6388

[codesandbox-ci[bot]]

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Sandbox	Source
example-instantsearch-getting-started	Configuration
example-react-instantsearch-getting-started	Configuration
example-react-instantsearch-next-app-dir-example	Configuration
example-react-instantsearch-next-routing-example	Configuration
example-vue-instantsearch-getting-started	Configuration