Speed up linkcheck by checking in parallel with 'cpu-count' processes

Author: hugovk

docs/Makefile

@@ -142,7 +142,7 @@ changes:
 	@echo "The overview file is in $(BUILDDIR)/changes."
 
 linkcheck:
-	$(SPHINXBUILD) -b linkcheck $(ALLSPHINXOPTS) $(BUILDDIR)/linkcheck
+	$(SPHINXBUILD) -b linkcheck $(ALLSPHINXOPTS) $(BUILDDIR)/linkcheck -j auto
 	@echo
 	@echo "Link check complete; look for any errors in the above output " \
 	      "or in $(BUILDDIR)/linkcheck/output.txt."