gnucash-docs: update to 3.4.

 3.4    - 30 December 2018
        o Convert help documents to use XInclude instead of SYSTEM entities
        o Convert Guide and its translations to use XInclude instead of
          SYSTEM entities
        o Drop old DocBook xsl 1.75.2 stylesheet
        o Base gnucash custom style sheet on DocBook xsl 1.79.2
        o Add DocBook xsl v1.79.2
        o Extract common entity declarations into a custom DTD
          This custom DTD is based on the DocBook V4.5 DTD and is intended
          to be used by all our documentation xml files.
        o Update to DocBook XML spec 4.5


(wiz)