Adds new Docs Style Guide.

Adds a new style guide for docs. This is meant to be the definitive reference for all questions of how to write and style our docs. Includes changes to mkdocs yml and moves style info from docs README, adding a link instead. Made a couple edits to the README too, while I was in there.
mmm, I think you need to tone this down - this reads too much like 'we don't want you to write unless you are a professional writer', when really, we'd rather good information, poorly written, than poor or no information, written precisely to the standard.