how to organize static assets in the filesystem tree

[parmentelat]

loosely related to #1250:

while we're at it talking about static assets, I take this chance to outline another possible improvement as compared to jb1
right now I have some books that are made of a non-trivial fs tree; I mean not all the source notebooks are located in unique folder; think for example of a book made of tens of programing exercises, each of which is made of a main README and related artefacts (media, data, ...)
for convenience, this use case maps into an fs tree with, say, one folder per exercise (all of them being in turn possibly organized into subfolders if need be)

in such cases when using sphinx and jb1, I think I had to group all the static assets into a unique toplevel _static folder

I guess my point here is, it might help to

• have mystmd let users put their static assets anywhere in the tree and not necessarily at the toplevel - as it would help keep related stuff together
• regardless, whether or not this feature is available (I can understand if it's not deemed high priority of course), the documentation could ideally have a section 'how do I manage and refer to my static assets' so that users know right away how to deal with this sort of questions

with jb1, it felt like many aspects, such as this one, were understandably not developed in the jb1 doc because they actually pertained to sphinx, I guess what I'm trying to say is, that is changing with mystmd rebuilding the whole stack from the ground up