Added a lightweight "core" Makefile entry

`make core` now runs a lightweight doc build process that doesn't include the
"api" target. The "html" target now just references "core" and "api".

This patch is a candidate for backporting to 1.x.

docs/Makefile

@@ -52,7 +52,9 b' dist: html'
 	cp -al build/html .
 	@echo "Build finished.  Final docs are in html/"
 
-html: api
+html: core api
+
+core:
 	mkdir -p build/html build/doctrees
 	$(SPHINXBUILD) -b html $(ALLSPHINXOPTS) build/html
 	@echo