Docs: Algolia search

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As suggested by @keynmol:

https://docsearch.algolia.com/apply/

Docusaurus has a native integration as well: https://v1.docusaurus.io/docs/en/search

This will help users search the docs more effectively than ctrl+F, and with that, we can maybe split the docs into separate pages. I was thinking to bring Airstream docs onto the website, but that's a separate issue.