go-tour: move programs and article file into new content subidirectory
Also put the programs into a subdirectory of prog, prog/tour.
This layout will keep things tidier as we add new tours.

At least for now, the images stay where they are because they are served
directly by the web server from the directory named static in the root.

R=adg
CC=golang-dev
https://golang.org/cl/13367049
70 files changed