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.~~
The integrate_chapters branch was merged into master in [pull request #26](https://github.com/angrave/sysbuild/pull/26) in sysbuild. See the pull request to view details and implementation of this integration.

Merging this pull request should not break anything in this repo, apart from following some steps when uploading new videos/content. The html files have been left intact so that the videos can be accessed independent of sysbuild.