Add include_html flag to generation of plugin REST API docs

ref #5009
https://pulp.plan.io/issues/5009

templates/docs/docs/Makefile.j2

@@ -42,7 +42,7 @@ clean:
 	-rm -rf $(BUILDDIR)/*
 
 html:
-	curl -o _static/api.json "http://localhost:24817/pulp/api/v3/docs/api.json?plugin={{ plugin_snake }}"
+	curl -o _static/api.json "http://localhost:24817/pulp/api/v3/docs/api.json?plugin={{ plugin_snake }}&include_html=1"
 	$(SPHINXBUILD) -b html $(ALLSPHINXOPTS) $(BUILDDIR)/html
 	@echo
 	@echo "Build finished. The HTML pages are in $(BUILDDIR)/html."