First cut at DocBook spec

Hi folks,

I think the last major stumbling block to publishing 5.1 is that we
have to have a spec document for it. The whole toolchain that built
the 5.0 spec seems to be in a shambles (no one's fault but my own, I
don't do XSLT 1.0 anymore).

So I hacked at it a bit. Here's my first cut. Still needs work but
maybe starting to get there. Comments, etc. most welcome.