The Apache Isis website and documentation are written using Antora and Asciidoc(tor):
Antora is the static website generator
Asciidoc is a markup syntax and Asciidoctor the tools to convert the markup into different formats.
HTML is the format generated by Antora, but other outputs such as PDF are also supported.
Details of installing Antora and building the website/documentation can be found here.
Some of the documentation is automatically generated from the code itself. Details on these generators is described on the tooling page.