Declarations not hyperlinked

[fpvandoorn]

In the type of some declarations, other declarations are not properly hyperlinked. For example
sign_apply does not link to SignType.sign.

Related to #104, which is about missing hyperlinks in markdown. Two examples in that issue aren't actually about markdown (Nat.toFinset_factors and WfDvdMonoid)

[kmill]

The sign_apply example is mostly a Lean pretty printing issue, and I think it should be fixed by leanprover/lean4#6085

The other ones involve fixing how docgen handles the terminfo. It needs to look to see if the getAppFn is a const, not that the expression itself is a const, like in the infoview. I did some experiments and it's a bit subtle to get right, since it can cause elements such as parentheses to become linkified.