Basic keyboard accessibility CSS tweaks

Mostly doubling-up :hover styles to also cover :focus, as a first step
to making the framework more keyboard-friendly.
Additionally, fixed two small markup issues in the docs/examples to
make the "Learn more" large primary button-styled links
keyboard-focusable (as without href they're treated as non-tabable
anchors).

less/labels-badges.less

@@ -35,10 +35,12 @@
   }
 }
 
-// Hover state, but only for links
+// Hover/focus state, but only for links
 a {
   &.label:hover,
-  &.badge:hover {
+  &.label:focus,
+  &.badge:hover,
+  &.badge:focus {
     color: @white;
     text-decoration: none;
     cursor: pointer;