Make - docs - Move opening the browser into a separate command.

Helps when triggering the doc build many times without opening yet
another browser tab each time.

Author: jomey

Makefile

@@ -71,6 +71,8 @@ docs: ## generate Sphinx HTML documentation, including API docs
 	# done
 	$(MAKE) -C docs clean
 	$(MAKE) -C docs html
+
+open-browser:
 	$(BROWSER) docs/_build/html/index.html
 
 servedocs: docs ## compile the docs watching for changes