Configurables are entities that came up in my research. They have ConfigClass attributes, but don't have run methods, so they're not technically tasks. They end up being values of config fields, which is why I ended up having to support them. I don't have an example off the top of my head. It may be that they won't get used in the future, in which case it'll be easy to drop that section in the future.
Yes, your intuition is right on why I went with an indices section rather than putting this page front and centre. Perhaps the time is right to just build out the structure of the Processing and Frameworks sections of the docs. After all, we can start populating these docs if we don't have the starting points.
I might prefer to do that after I finish integrating docs builds with scons and the stack-os-matrix job so that development is a bit more straightforward.