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 
Should be putting each details into an unordered list (<code>&lt;ul&gt;&lt;li&gt;</code>). Can hide the list with <code>class="list-unstyled"</code>. That is to provide fallback structure for screen readers where they don't have support for details/summary.