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/thumbnails.less

@@ -33,8 +33,9 @@
   .box-shadow(0 1px 3px rgba(0,0,0,.055));
   .transition(all .2s ease-in-out);
 }
-// Add a hover state for linked versions only
-a.thumbnail:hover {
+// Add a hover/focus state for linked versions only
+a.thumbnail:hover,
+a.thumbnail:focus {
   border-color: @linkColor;
   .box-shadow(0 1px 4px rgba(0,105,214,.25));
 }