Added entry for Microsoft Dev Box

Added entry for Microsoft Dev Box, which goes to public preview on Aug 1.

Author: RoseHJM

bread/toc.yml

@@ -1086,3 +1086,6 @@
   - name: Azure Spring Apps
     tocHref: /azure/spring-cloud/
     topicHref: /azure/spring-cloud/index
+  - name: Dev Box
+    tocHref: /azure/dev-box/
+    topicHref: /azure/dev-box/overview-what-is-microsoft-dev-box