add htmlsingle target (#30287)

For whatever reason, building the 'singlehtml' version of
the docs is much much faster than building the normal html
version.

docs/docsite/Makefile

@@ -20,10 +20,14 @@ all: docs
 
 docs: clean htmldocs
 
-htmldocs: testing keywords modules staticmin cli config
+generate_rst: testing keywords modules staticmin cli config
 
+htmldocs: generate_rst
 	CPUS=$(CPUS) $(MAKE) -f Makefile.sphinx html
 
+singlehtmldocs: generate_rst
+	CPUS=$(CPUS) $(MAKE) -f Makefile.sphinx singlehtml
+
 webdocs: docs
 
 #TODO: leaving htmlout removal for those having older versions, should eventually be removed also