Implement a skiplink

Part of #1106 (closed) (read this issue for access to the full audit and recommendations).

What is a skip link

Note that it would likely require a reordering of the HTML elements, in particular moving the sidebar after the <router-view></router-view>` to ensure content is always first on the page

Note that we may also need to implement some focus-related tweaks to reset the focus when a link is clicked (cf https://daverupert.com/2019/01/accessible-page-navigations-in-single-page-apps/).

Audit section (Section 2.4.1):

A mechanism should be available to skip content that is repeated throughout the website. This can be done by means of a skiplink. The skiplink can be hidden with CSS and displayed only when it receives the focus, and it should be the first link on the page that receives the focus.