This documentation section is about creating a new PDF document and its contents from scratch. So think creating an invoice and not combining pages from multiple documents.
There are basically two ways to achieve this with HexaPDF:
By using the high-level document layout functionality available in the HexaPDF::Layout module standalone or through HexaPDF::Composer.
This is the recommend way to create documents. See Automatic Document Layout for details.
By manually creating and building the contents yourself using the low-level HexaPDF::Content::Canvas API.
This is possible but for complex documents you will probably re-implement all the logic that is already available in the document layout functionality of HexaPDF.