docs: Add ARC documentation TOC file

Bug: 30114799
Change-Id: Ib4e3c5b255c9e1e9c2894b3703deb1d38f252fba

docs/html/topic/arc/_book.yaml

+toc:
+- title: Optimizing Apps for Chromebooks
+  path: /topic/arc/index.html
+- title: App Manifest Compatibility for Chromebooks
+  path: /topic/arc/manifest.html
+- title: Loading Apps on Chromebooks
+  path: /topic/arc/sideload.html
+- title: Chrome OS Device Support for Apps
+  path: /topic/arc/device-support.html
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