I don't know about "Docbook-Website" in particular, but The Linux Documentation Project ([tldp.org ]) uses a DocBook based XML DTD; see their Author's Guide.
There is a section that discusses why they use DocBook and XML [tldp.org].
Actually, a bit of a search turned up the "Docbook-Website" home page; it's now called "docbook-xml-website" [packages.debian.org]
It's evidently based on the work of the illustrious Norm Walsh ([nwalsh.com ], but his page redirects you to
the Docbook project at Sourceforge [sourceforge.net]
Check out the documentation page at that project:
It's full of good stuff that should help get you started.
My two cents: in general I'm in favor of using XML where it adds value, but it can also add significant complexity (and complexity==cost). So ask questions. Do you have a complete toolset lined up? For example, will you need an XML editor, and if so, is there one that is within your client's budget? And the most basic: what value will using XML add?
Best wishes for the project.