Add links to type aliases

[jfmengels]

Hi 👋

This adds links to type aliases.

Background

I noticed a while back that some types get links (in the type of a function for instance), while some of them don't. I wondered why, and it looks it's as simple as custom types get links, while type aliases don't.

You can see an example here: https://package.elm-lang.org/packages/mdgriffith/elm-ui/latest/Element
Element is a type alias, and it doesn't show up as a link in the signature of none below it.

Additional information

If/when this gets merged, I'll make a similar PR to dmy.elm.fr and to elm-doc-preview so that they can also benefit from the change.

I could not test this change out as I didn't know how to get the server running. I did test it however with elm-doc-preview (see my fork and branch here).

I can't think of any drawbacks to include these links, but if you do see any, please let me know!