Docs organized as a "set"

Hi Peter. I finally got a look at the "set" organization of the Postgres
docs. Looks pretty nice; there is a tradeoff between the "monolithic
ToC" from before and the more organized look of the "books within a
set".

I have no objection to using the "set" style (barring unforseen
difficulties); my recollection is that you are recommending that, or
were you just exploring the idea?

- Thomas