Writing up XDG base directory standard non-compliance

[ivanov]

This is a recurring and now cross-project concern where some users currently wish that Jupyter followed the XDG base directory standard, but the projects do not necessarily do that.

I've already written up some proposed language on how to work around our non-compliance on a recent JupyterLab issue about this, and then I saw that, at least according to the docs, we do partially use XDG directories, so maybe that's the place to document this.

[minrk]

I think the jupyter directories docs is the right place to discuss the relationship to XDG. IIRC, the pending path deprecation and switch to appdirs is still not documented there.