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/carousel.less

@@ -97,8 +97,9 @@
     right: 15px;
   }
 
-  // Hover state
-  &:hover {
+  // Hover/focus state
+  &:hover,
+  &:focus {
     color: @white;
     text-decoration: none;
     .opacity(90);