Build DocX exporter

Refs https://redmine.teklia.com/issues/7497

We need a new exporter using shared BaseExporter, and exposing a new DocXExporter class in worker_export.docx module. A new command worker-export-docx is then created in pyproject.toml, targetting this class boot code. A new worker must be declared in .arkindex.yml

The worker must support the following parameters:

• optional folder type (default to folder)
• optional element type (default to page)
• optional line type (default to text_line)
• optional boolean merge (default to false)