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/scaffolding.less

@@ -23,7 +23,8 @@ a {
   color: @linkColor;
   text-decoration: none;
 }
-a:hover {
+a:hover,
+a:focus {
   color: @linkColorHover;
   text-decoration: underline;
 }