Reference-style links don't work in keypoints

[gcapes]

I appreciate this probably isn't a common use case, but if you try to link in a keypoint like this:

[link text][link-reference]

with this elsewhere in the markdown file,

[link-reference]: https://www.thelink.com

it doesn't render as a link. You just see the markdown.

However if you use an inline link it works, but you end up with repetition in your markdown file:

[link-text](https://www.thelink.com)

[zkamvar]

N.B. This will not be an issue in The Carpentries Workbench as the keypoints are part of the narrative text.