Explain purpose of `files`, `code`, and `data` folders

[gcapes]

This is partly discussed in #234, but I would like a note in the rendered material to explain what types of files should go in the files, data, and code directories? Different lessons seems to take different and mixed approaches to this. Perhaps files could be dropped because it's fairly ambiguous, but the other two would benefit from brief documentation.

[gcapes]

Is there a better place for this discussion to happen? Just there have been no comments and it's been 4 months :(

[brownsarahm]

I think this is the right place, but because of the workbench transition this repo is approaching retirement and a lot of attention is over there, not here, for maintainers and the infrastructure team.

My understanding is that these are mostly legacy features.