docs: fold the prog.sh scripting from makehtml into htmlgen itself.

docs: fold the prog.sh scripting from makehtml into htmlgen itself.
This allows us to drop some crufty scripting and provides a firmer
footing for building better tools for preparing documents with source
code inside.

Also eliminate line numbers from the examples and text.

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/4650069