Add an autohtml target to docs/Makefile.

This runs the convenient sphinx-autobuild web server for docs development.

cranelift/docs/Makefile

@@ -4,6 +4,7 @@
 # You can set these variables from the command line.
 SPHINXOPTS    =
 SPHINXBUILD   = sphinx-build
+SPHINXABUILD  = sphinx-autobuild
 PAPER         =
 BUILDDIR      = _build
 
@@ -56,6 +57,9 @@ html:
 	@echo
 	@echo "Build finished. The HTML pages are in $(BUILDDIR)/html."
 
+autohtml: html
+	$(SPHINXABUILD) -z ../meta -b html -E $(ALLSPHINXOPTS) $(BUILDDIR)/html
+
 dirhtml:
 	$(SPHINXBUILD) -b dirhtml $(ALLSPHINXOPTS) $(BUILDDIR)/dirhtml
 	@echo