Sysbuild integration

`sys.json` exposes an "API" for [sysbuild](https://github.com/angrave/sysbuild) which allows seamless integration of video chapters and code playground activities. Video docs, challenges, etc. can be written in markdown and placed in the relevant chapter folder.

See the [integrate_chapters](https://github.com/angrave/sysbuild/tree/integrate_chapters) branch on sysbuild to see details about/implementation of this integration.