Restore custom reference manual titlepage

The new gradle plugin that generates documentation creates a different title page than version 3.1.

The previous ant build supported a custom titlepage.xml file (something like [1]).

The main problem is the large list of authors that dominates the first page. Perhaps removing the <author> elements from the docbook source might be another way to fix this?

[1] https://github.com/cwensel/docbook-framework/blob/master/build-docbook.xml#L160