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/mixins.less

@@ -506,7 +506,7 @@
   .reset-filter();
 
   // in these cases the gradient won't cover the background, so we override
-  &:hover, &:active, &.active, &.disabled, &[disabled] {
+  &:hover, &:focus, &:active, &.active, &.disabled, &[disabled] {
     color: @textColor;
     background-color: @endColor;
     *background-color: darken(@endColor, 5%);