Make open positions easier to find
Right now this information is buried on the "Staff and Contractors" page, which does not suggest by its title that it might have hiring information in there. Also, this page is in a sub-menu of the main navigation with no links on the main page. As a result, it is not easy to find.
To address that, this commit changes the name of the page to "Careers in KDE" and links to it from the main page.