_site.yml

name: "site"
output:
  html_document:
    theme: sketchy
    bootstrap_version: 4
    theme_variables:
      primary: pink
      font-size-base: 1.5rem
navbar:
  title: "My Website"
  type: "inverse"
  left:
    - text: "Home"
      href: index.html
    - text: "About"
      href: about.html
      menu:
        - text: "Sub_1"
          href: sub1.html
        - text: "Sub_2"
          menu:
            - text: "Sub_2_below"
              href: sub2.html
  right:
   - text: "Home"
     href: index.html
   - text: "About"
     href: about.html
     menu:
       - text: "Sub_1"
         href: sub1.html
       - text: "Sub_2"
         menu:
           - text: "Sub_2_below"
             href: sub2.html

render-site.R

library(rmarkdown)

index <-
'---
title: "My Website"
---

Hello, Website!'

about <-
'---
title: "About This Website"
---

More about this website.'

sub_part_1 <-
'---
title: "About This Website"
---

sub_1: This is right below the About page.'

sub_part_2 <-
'---
title: "About This Website"
---

sub_2: This is 2 levels below the About page.'

writeLines(index, "index.Rmd")
writeLines(about, "about.Rmd")
writeLines(sub_part_1, "sub1.Rmd")
writeLines(sub_part_2, "sub2.Rmd")

# TODO:
# 1. Account for navbar height
# 2. submenu is broken for some themes (e.g., sketchy)?
render_site()