clarify language/nav for getting started with DrupalLadder.org

Overview

Clarify language, nav, etc. around getting started with learndrupal.org. Short term: minimum amount of work needed to guide someone through the things they need to do.

Next tasks

Add 'how to use' description to the ladder

Background

There are various audiences with specific needs:
- lesson takers
- lesson creators (add/test/edit)
-- instructions on getting right into steps and putting references out of the stream
- organizers of learn and issue sprints
- code contributors
-- creating features
-- theming
- content managers
- site administrators

(more underway in this goodoc; please take a look and add comments)