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 
{{{i18n "tmpl-about"}}} should be changed to just {{{i18n "about"}}}. There is no need for tmpl-about with about having existed for a long time.