Makefile: Consolidate documentation targets, and add "htmlclean"

* removed docs and docs-only targets

* apidoc       : generate apidoc

* htmldoc-only : build apidoc, build html

* htmldoc      : build apidoc, build html and open in browser

* htmlclean    : remove documentation build artifacts

Makefile

@@ -39,19 +39,20 @@ test_docker:
 
 apidoc:
 	cd docs && $(MAKE) apidoc
-	@echo ""
-	@echo "To view the PDF documentation open: docs/_build/html/index.html"
 
-htmldoc:
+htmldoc-only: apidoc
 	cd docs && $(MAKE) html
-	@echo ""
-	@echo "To view the PDF documentation open: docs/_build/html/index.html"
 
-docs-only: htmldoc
+htmlclean:
+	cd docs && $(MAKE) clean
 
-docs: docs-only
+htmldoc-open:
+	@echo ""
+	@echo "To view the HTML documentation open: docs/_build/html/index.html"
 	open docs/_build/html/index.html || xdg-open docs/_build/html/index.html
 
+htmldoc: htmldoc-only htmldoc-open
+
 pdfdoc:
 	cd docs && $(MAKE) latexpdf
 	@echo ""