Support Docx via PDF conversion

Docx documents can easily be converted (or printed) to PDF.

The advantage of this process is that the printing process generates proper layout and visualization components as pages, bounding boxes, etc.
The disadvantage is that the process is a bit slower than reading the native Docx format, and that all the semantic content must be re-inferred (e.g. section headers, etc)

Reading Docx via PDF conversion will be one of the possible ways of using Docx document as input. See #105 for the native fast parsing.