Second pass on introductory materials

This is a further edit of the introductory material.

Again focus is one removing the repetitive sections, making it more succinct and cleaning up some of the language. 

I think the Working with Docker page needs more work. It needs to tell a clearer story than it does now. I especially pulled the `docker commit` section - we want to de-emphasize this workflow in favor of `docker build` and Dockerfiles.