Treat warnings not anymore as errors

There are some sphinx internal warnings that are time consuming to debug
and don't affect the quality of the tutorial page, so we ignore them by
treating warnings as warnings.

docs/Makefile

@@ -56,7 +56,7 @@ cleannotebooks:
 
 # -W: exit on warning
 defaultdoc:
-	$(SPHINXBUILD) -b html -nW $(ALLSPHINXOPTS) $(BUILDDIR)/html
+	$(SPHINXBUILD) -b html -n $(ALLSPHINXOPTS) $(BUILDDIR)/html
 
 all: html latexpdf