Add examples showing generated files

[mwichmann]

Many have had problems when files which should be generated (if needed) during the build are needed by parts of the build. Usually they end up reaching for Depends and it doesn't work as they expect. Show how to get SCons to understand the existence of a generated file if it's done by a builder (or Command) that doesn't have direct knowledge that the file is being made (that is, it's not part of an emitter).