Add a autobuild-quick target that only rebuilds HTML.

Doc/Makefile

@@ -171,6 +171,10 @@ autobuild-dev:
 	make update
 	make dist SPHINXOPTS='-A daily=1 -A versionswitcher=1'
 
+# for quick rebuilds (HTML only)
+autobuild-html:
+	make html SPHINXOPTS='-A daily=1 -A versionswitcher=1'
+
 # for stable releases: only build if not in pre-release stage (alpha, beta, rc)
 autobuild-stable:
 	@case $(DISTVERSION) in *[abc]*) \
@@ -178,3 +182,4 @@ autobuild-stable:
 		exit 1;; \
 	esac
 	@make autobuild-dev
+