Subchapters...?

I really like the ease of building these documentations.

Looking at the notes collection I had a thought. PCA (for example) is always the same regardless of the data and one can introduce it with any basic ML dataset. However, the trainings and materials are for people who are not ML experts and might not have a good grip on abstracting a method from a demonstration on the iris dataset. Still, the basic ideas are very useful but using the dataset of the particular audience in training is important.

Currently we could/do reuse chapters and change the datasets for the same workflows, which almost duplicates the number of chapters in the TOC. How about adding a subchapter that would appear as an openable group in the sidebar and could contain the specific demos or more complex workflows for the same topic...? Do you think such an addition can be useful? Is it a lot of work to add?