Environment documentation

Clarify how r10k works with dynamic environments and how to test using an environment.
Correct capitalization of r10k in a few places.
Nowhere higher in the quickstart guide is creating a branch (and hence new environment) mentioned than the summary. To move it higher would require integrating branch creation into the quickstart guide, which would be a much more significant rewrite. I think since this section replaces the existing instructions for using an environment, it's best left here until the guide is rewritten as a whole.