nbconvert is not yet installed at time of doc generation.

Patch by Marius Brehler
--- docs/Makefile
+++ docs/Makefile
@@ -61,8 +61,8 @@ source/config_options.rst:
 	@echo "Created docs for config options"
 
 ipynb2rst:
-	jupyter nbconvert --to rst source/customizing.ipynb --FilesWriter.build_directory=source
-	jupyter nbconvert --to rst source/nbconvert_library.ipynb --FilesWriter.build_directory=source
+	../scripts/jupyter-nbconvert --to rst source/customizing.ipynb --FilesWriter.build_directory=source
+	../scripts/jupyter-nbconvert --to rst source/nbconvert_library.ipynb --FilesWriter.build_directory=source
 	@echo "Converted notebooks to rst"
 
 dirhtml: