Font Awesome
<i class="fa fa-rocket" aria-hidden="true"></i>
Is how to show a Font Awesome icon. Which looks like this:
But there is a dependency and some bugs. Font Awesome only seems to work if there is an actual image on the page. For example, this is an edit icon from a normal svg file. 
. Remove that and your Font Awesome icon will disappear. And FA doesn't work if I close the tag properly. And the i element is duplicated if you put a br tag near it. Note: the image file doesn't even have to exist, it's just that the image processing code is not called unless there is a [[File:]] tag in the wikitext.