Make the material creation tasks more 'bite sized'

[s-weigand]

Is your feature request related to a problem? Please describe.
As is now, the assigned material writing is in the form of chapters, which require a lot of work at once and can be overwhelming. IMHO, this leads to slower creation cycles (ready for review) and even (at least in my case) deferring of writing it at all.

Describe the solution you'd like
If we size down the tasks to sections or subsections, the amount of work the material creators need to apply at once will be considerably less. Also, this will be more rewarding, since you can check off a checkbox, which at least for me is always a good feeling of accomplishment.
For the maintainer this might lead to more work since it can lead to merge conflicts and jupyter notebooks aren't a git friendly format, but with reviewnb this can hopefully be solved in an easy manner (not directly working on the JSON blob of a notebook).