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.
tell em how to make this fancy pants `code font` - otherwise you'll get someone using html and setting the font using a css style - and they will be right....