add a skiplink to jump to the main content of the page

To the benefit keyboard users.

the tabindex=-1 is a hack to make sure that focus is actually transferred to the correct part of the page so that  a next tab is within the context of the main content of the page and not the context of the page  (on some brosers such as chrome this is broken). I don't like having the focus ring appear on the `main` element as a result of this hack, so hide that using `outline:none`