docs: Add ARC documentation TOC file am: 12901253 am: f7355ded

toc:

- title: Optimize Apps for Chromebooks

  path: /topic/arc/index.html

- title: Loading Apps on Chromebooks

  path: /topic/arc/sideload.html

- title: Chrome OS Device Support for Apps

  path: /topic/arc/device-support.html

- title: App Manifest Compatibility for Chromebooks

  path: /topic/arc/manifest.html

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