The documents use the asciidoc format, see:
Asciidoc version 8.6.3 is a requirement together with whatever dependencies it needs and specifically docbook, w3m, fop and graphviz.
Apache Maven together with Make are used to unpack the pieces of the manual and to execute asciidoc on them.
To build the documentation, use:
mvn clean install
To perform a faster check of the document structure and to see if snippets and images are correctly resolved, use:
mvn clean compile -Dcheck
which will only produce html for offline use and text outputs (including the manpages).
Note that this profile can’t be used together with the package goal.