Set Python path in Sphinx config to pick up local sgkit in notebooks

docs/Makefile

@@ -1,6 +1,8 @@
 # Minimal makefile for Sphinx documentation
 #
 
+PYPATH=${PWD}/..
+
 # You can set these variables from the command line, and also
 # from the environment for the first two.
 SPHINXOPTS    ?=
@@ -22,4 +24,4 @@ clean:
 # Catch-all target: route all unknown targets to Sphinx using the new
 # "make mode" option.  $(O) is meant as a shortcut for $(SPHINXOPTS).
 %: Makefile
-	@$(SPHINXBUILD) -M $@ "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) $(O)
+	@PYTHONPATH=${PYPATH} $(SPHINXBUILD) -M $@ "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) $(O)