Add guidelines for actions in Ember

Without the `href="#"` links do not get a hover state. Apparently, they also cannot be tapped on a touch device.
It will not work as you expect that is why the first rule is to use the button instead.

Ember's actual links use a special `{{link-to}}` and they work as expected. Actions do not generally change your route but even if they do, they will not work properly when opened in a new tab.

It is possible that we should change the previous guideline to avoid using `a` when using actions.