Also update the big example to the new template system.
There are a number of other examples that should be
extracted; this CL serves as an introduction to the
approach.
R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/4923043
No longer do we generate HTML from it; instead the input
file is already in HTML but has template invocations to
extract programs from other files.
Delete htmlgen, which is no longer needed.
Add tmpltohtml, which runs the templating code.
R=golang-dev, dsymonds, adg
CC=golang-dev
https://golang.org/cl/4699041
