Hugo is the main site generator used to put together the site. You can think of it as a build step, acting as a "transpiler" to shift different files (Markdown files containing textual content, HTML files containing layout templates, and JSON files containing data), putting all of the contents together into one cohesive website, which then outputs as files which can be directly uploaded to the website. Using Hugo is critical, and has helped simplify putting together the website significantly.
Bulma - comprehensive CSS framework
Bulma is used in the website in order to simplify page layout and theming. It comes with a good theme out of the box, and that theme is tweaked slightly with Slugbotics's colors through the website theme repository.
HTML/CSS/JS - the building blocks for websites
Useful resources for learning HTML/CSS/JS if you are a beginner to the subject.
Hugo Tutorial Series by Mike Dane - this teaches you Hugo, which is the static site generator used to template pages and generate a copy of the site.
CSE 183 Spring 2020 - most content here not relevant, but lectures on HTML/CSS and Bulma may be useful.