Tidy doc processing

This tidies the various image files into a new ```graphics``` directory, removes a bunch of no-longer-used image files, leaves the ```doc``` directory tidy at all times by doing pdf and html document generation in sub-directories, does appropriate image scaling for the html documents (and also supplies some simple CSS styling for them). Some scripting has been moved back out of the Makefile for convenience (but ```make``` still properly recognizes when a document needs re-making).  I think I'm now done pfaffing about with the ```doc``` dir.