Improve accessibility by adding a skiplink and label to navigation

[KreerC]

This PR seeks to improve the accessibility of Dokuwiki pages using the bootstrap3 template by adding a skip link which allows users with screenreaders to skip any navigation whatsoever and get right to the content. It also adds a descriptive aria-label to the collapse navigation button so it's easier for users to make sense of it's function. For this, it was necessary to add a new translation label to the template. I've translated it to German already.