Thanks for links, I already read this pages, but I am still confused how to achieve this. I tried to add index.md and _index.md with headless=true or without. I still get list of pages for url http://localhost:1313/menu/
I’ve just found a solution that works for me. This is the content of _index.md:
+++
url = "/"
+++
The advantage is that the url to this branch is not displayed in sitemap.xml(!).
However, I don’t know if this solution will work permanently because the same URL is used for multiple pages. A headless option for branches (_index.md) would be safer.