A11y ☝️😎👍

[jminguely]

• [x] Add access key to main element in every page
• [x] Generate a complete menu (don't have to contain all the items, more a exemple purpose) with only <nav role="navigation"><ul><li><a> hidden for the rest of us but ARIA visible.
• [x] Every section (search, navigation, content, related-content, side-navigation, footer) should have a h2.sr-only with the correct name.
• [x] Every button must have a aria-label=""

[tonifisler]

@jminguely check issue #249 too, we need alt attributes on all images.