Organize experiments/ (#2261)

Removed old recipes folder.
Moved navigation and arena experiments into `experiments/recipes`
Added the README.md from my `experiments` branch as a DESIGN.md

[Asana
Task](https://app.asana.com/1/1209016784099267/project/1210348820405981/task/1211107824176610)

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Co-authored-by: Jack Heart <jack@softmax.com>
Co-authored-by: graphite-app[bot] <96075541+graphite-app[bot]@users.noreply.github.com>