A new beta MkDocs theme for Docker Docs.

A new beta MkDocs theme for Docker Docs with:

 - Docker IO colour scheme;
 - Clear top, left and main site navigation bars;
 - Breadcrumbs;
 - Docker IO footer & links.

Please note that this theme is kept explicitly non-responsive in order to:
Minimise the amount of time possible/expected changes will take for the initial release.

Docker-DCO-1.1-Signed-off-by: O.S. Tezer ostezer@gmail.com (github: ostezer)