Here is a video from this years WWDC showing all the cool stuff you can do with DocC: https://developer.apple.com/videos/play/wwdc2021/10166/

I personally think it was a great idea to open source this, since everyone's main complaint about DocC were rooted in its proprietary nature [0][1]

[0]https://news.ycombinator.com/item?id=27686069

[1]https://news.ycombinator.com/item?id=27496016