Add Slovak translation to documentation

Author: theyosh

docs/_includes/topbar.html

@@ -75,6 +75,7 @@
       { name: 'Norwegian', key: 'NO' },
       { name: 'Polish', key: 'PL' },
       { name: 'Portuguese', key: 'PT' },
+      { name: 'Slovak', key: 'SK' },
       { name: 'Spanish', key: 'ES' },
     ];