add macOS support for the 'docs' make target (#37)

Makefile

@@ -91,7 +91,7 @@ release:          ## Create a new tag for release.
 docs:             ## Build the documentation.
 	@echo "building documentation ..."
 	@$(ENV_PREFIX)mkdocs build
-	URL="site/index.html"; xdg-open $$URL || sensible-browser $$URL || x-www-browser $$URL || gnome-open $$URL
+	URL="site/index.html"; xdg-open $$URL || sensible-browser $$URL || x-www-browser $$URL || gnome-open $$URL || open $$URL
 
 .PHONY: switch-to-poetry
 switch-to-poetry: ## Switch to poetry package manager.