Display Haddock diagnostics

## Is your enhancement request related to a problem? Please describe.

Haddock produces various warnings that are somewhat useful. For example, it will tell you if it cannot figure out where to link something to, which often indicates a typo.

These warnings are rarely noticed, because the only time you see them is when you run `cabal haddock` (and the format is terrible). Surfacing them in HLS would be pretty helpful.

## Describe the solution you'd like

Haddock warnings (and errors?) appear in HLS like normal diagnostics.

## Additional context

I have no idea how hard it is to get these. I think we are running haddock (if `documentation: true` is set), so presumably we should be able to get the warnings too.

In addition, I notice that the warnings printed out by haddock on the CLI don't even have locations (!!!), so we might have to work on Haddock to get the information we need.