Rewrite some early sections of the dependency injection guide

Some editing to #1293 - really I've only hit the introduction and first section here. Further thoughts:

* The audio example I think is fundamentally a sound one (ha!).
* The object-management guide should be removed. I don't think it adds anything to what can be covered under the dependency injection guide.
* The logger example is also a good one. I suggest removing the literal `console.log` statements from it though.

And the last two sections could also use a solid editing pass.