[#389] Proper haddock check in ci

Description

Problem: Haskell.nix moved haddock building to a separate derivation at some point in time, thus we weren't testing it in CI.

Solution: Add dedicated haddock check to test-extras stage in CI.

Related issue(s)

Resolves #389 (closed)

✅ Checklist for your Merge Request

Related changes (conditional)

• Tests (see short guidelines)

  • If I added new functionality, I added tests covering it.
  • If I fixed a bug, I added a regression test to prevent the bug from silently reappearing again.

• Documentation

  • I checked whether I should update the docs and did so if necessary:
    • Root README and other README.md files
    • Haddock
    • docs/
  • I updated changelog files of all affected packages released to Hackage if my changes are externally visible.

Stylistic guide (mandatory)

• My commits comply with the following policy.
• My code complies with the style guide.