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.
I think the "as small as possible" description for images may require some additional information (could be added at a later stage). Give the users some pointers; are we talking about dimensions or file size? Is SVG allready acceptable for the targeted audience?

The problem I see is that people without the right knowledge might "destroy" a useful graphic to a crappy image if they try to reduce file size and decide to convert to a lossy compression or resize the image the wrong way.

I *think* (but hey, I'm not a maintainer :)) that it's sometimes better to have people send their best quality source image (not talking about 36mp photos of course) and have someone of the Docker team look at it to convert it to the right format/dimensions.

Perhaps always have the original files sent separately so that Docker can create new versions/sizes of the images in case dimension have to be adjusted on design-changes of the documentation.

(Sorry for the lengthy comment. It's my daily business :))