The _site.yml specifies the basic info about your site and the navigation bar at top.

name: "my-website"
output_dir: "."
navbar:
  title: "My Website"
  left:
    - text: "Home"
      icon: fa-home
      href: index.html
    - text: "About"
      icon: fa-bullhorn
      href: about.html
output:
  html_document:
    theme: readable
    toc_depth: 3
    toc_float:
      collapsed: false