Changed "you would currently have to write" to "you would otherwise have to write".
Seems to make more sense this way since "currently" presupposes that someone new
to Angular would be coming from a different paradigm, which they may or may not be.
Closes#9428
Before, there we multiple overview docs:
- guide/overview
- guide/introduction
- guide/dev_guide.mvc
- guide/dev_guide.mvc.understanding_model
- guide/dev_guide.mvc.understanding_view
- guide/concepts
Now we have:
- guide/introduction: High level description of Angular with the key benefits but without code or any concrete concepts
- guide/concepts: explains all important concepts with a simple example and contains deep links to the other parts of the guide.
All the old information was moved into existing documents or deleted when they were duplicates.
