Here's how to do it with more recent versions of Hugo:

https://photostructure.com/coding/hugo-markdown-output/

(It includes the grandparent's head link suggestion, but it's not just "change .html to .md" because I'm old skool and as a wee nerd was told that URLs ending in .html or .php or whatever we're frowned on, so the above link's markdown is available by appending /index.md )