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.
Yes, I was thinking among those lines as well. Sure, we must prevent adding huge images to the repository, but at the same time prevent people destroying them for the sake of keeping them small.

It's probably not a big issue, don't think there are many images currently in the docs.