Design Document XML resurrected (partial revert of r19836)

I've resurrected the Design Document XML on a temporary basis.

My reason for doing this is that the current converted design.html
contains numerous errors in the markup - which there's really no reason
for, considering the source is a fully valid DocBook document.

Therefore, it is my intention to construct some XSLT to perform the
conversion to HTML in a well defined and rigorous fashion.

I have a mostly-working transformation already, by way of private
proof-of-concept. I'll tidy it up a little more, and commit it into the
doc/design directory. People can comment, and adjust the transformation
if necessary, then we can run it for a final time, replace the current
design.html with the XSLT-transformed one, and then delete doc/design again.

Max.