De-jargon-ify MVC explanation.

Prefer stating true things to metaphors. Metaphors won't help later, and
facts can at least give them a vocabulary and concepts to start putting
together.
split out separate 'rails architecture' page
process 'goal' text as markdown; improve stepfile docs
Merge pull request #298 from railsbridge/never-speak-mvc

De-jargon-ify MVC explanation.