@p4plus2 the Documents tab is intended as a navigator for, well ... Documents, ie things open in Geany. As such it only shows directories for open documents.
There is a "filebrowser" plugin that is intended for navigating the file system, and that is probably a better place for things like creating new files since it can access any directory, not just ones with files already open.
And the standard required for a plugin (even a core plugin like filebrowser) is lower than for getting it in Geany itself.