javadoc

The way to take code with documentation comments and produce a documentation website is with the javadoc tool.

There are multiple ways to run this¹, but if you have your code in the multi-module directory layout you can use --module-path and --module the same as javac does.

javadoc \
    -d output/javadoc \
    --module-source-path "./*/src" \
    --module one.piece

This will produce a directory full of HTML files that contain all the documentation from the specified modules.

This is what is used to make the official Java documenation as well as at least part of the documentation for most Java libraries.