/about-us/ is a section not a page. Thus, according to Zola, a section has a _index.md .. but an _index.md file is attach with an file.html. That's why we cannot take advantage of a markdown content..for a section content.
It's the same thing with the homepage.
So basically, homepage and all section content (e.g URL like this ROOT/section/ ) will have its HTML file.
