Install Ruby however is best for you. Living in maclandia I used the one that came with XCode. Brew is also good
in the root source directory astrochart.github.io run "bundle install" this should install all the necessary libararies listed in the Gemfile
Solve any problems resulting from your badly configured ruby install. EG I found this page helpful https://stackoverflow.com/questions/63729369/commonmarker-gem-cannot-be-installed-needed-for-jekyll-macos
Once everything is installed, tell jekyll to compile the page and start a local debug server
bundle exec jekyll serve
More clues about this process here https://docs.github.com/en/pages/setting-up-a-github-pages-site-with-jekyll/testing-your-github-pages-site-locally-with-jekyll