First of all, OxygenXML has a free trial license that will not watermark your output in any way.
Probably the quickest way.
Still, I am a bit curious why Docbook in particular - it is an intermediary format not meant for end user user consumption, I guess the idea is you want to then produce a nice looking PDF and HTML out of that? If that's the case, not a bad plan.
Also it's not a whole lot of work to write your own XSL stylesheet to do that, you just need to map:
complexType
and simpleType
and element
to db:section xs:name
to db:title
and xd:annotation
to para
then you can process this with free tools like saxon and docbook toolkit.