docs/code-style.md: Reference better docs for haddock markup
Problem: The haddocks docs on haskell.org/haddock aren't being updated.
Solution: Link to up-to-date docs on haskell-haddock.readthedocs.io/en.
Problem: The haddocks docs on haskell.org/haddock aren't being updated.
Solution: Link to up-to-date docs on haskell-haddock.readthedocs.io/en.