Web version of Design guide

The latest version of the mock-up is

https://w3c.github.io/wai-coga/coga-draft/guide/

Note, the content has yet to be added. I'm waiting for it to come out of 
the Review process we are now embarking on.

I have also created a GitHub issue to track the "official" design and 
coding of the design guide so it fits perfectly into the WAI website.

https://github.com/w3c/wai-website/issues/138

Steve