Add new Getting Started page

Modified content in start-en.hbs to match the new theme, 
Added page specific css in content/_base,scss
Modified colours in main/_base.scss and footer/_base.scss
Modified link colours in _variables.scss 
Providing only an image as a link is problematic. Also "Base theme" is not a helpful link description. It leads to documentation for the theme. There should be text-based link (which could include an image after if you like) but it should not be an image-only link.