[ADD] How we works page

...rce/_patterns/molecules/how-we-work/sidebar-menu.mustache

...terns/molecules/how-we-work/work-article-preview.mustache

...leguide/source/_patterns/molecules/simple_header.mustache

main/styleguide/source/_patterns/pages/how-we-works.mustache

...yleguide/source/_patterns/templates/how-we-works.mustache

main/styleguide/source/css/atoms/container.pcss

main/styleguide/source/css/atoms/heading.pcss