This was created using a template of an HTML GitBook style bookdown book, hosted on GitHub Pages. It is based on the one created with File, New Project..., New Directory, Book Project using bookdown in RStudio. It assumes you know how to use git/GitHub and R Markdown, and have some familiarity with the bookdown package. If you don't, the package author Yihue Xie's RStudio webinar Introduction to Bookdown (R Package) provides an excellent introduction. In addition, check out bookdown: Authoring Books and Technical Documents with R Markdown, also by Yihui Xie, both for the content and the format--it is itself a bookdown book.
As always, please consult the official guide to bookdown: https://bookdown.org/yihui/bookdown