The docs are built using Jekyll and are hosted by Github Pages.
To build them locally, you need to follow the Github Pages documentation and create the appropriate Gemfile.
Afterwards, just run bundle exec jekyll serve if you want to serve them locally and preview any changes you may make, or build them using bundle exec jekyll build.
Github pages automatically build the website, thus PR should concern only the source files (e.g index.md) and not the output of jekyll build (e.g index.html).