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.
Should links to other sections of the documentation be relative (`../../other-doc/`), absolute (`/docs/sections/other-doc/`) and should they contain the domain name? (`https://docs.docker.com/.......`)

Sorry for the technical remarks, I know this document is primarily about *style*