doc/haskell: (re-)init haskell manual section

This restarts a Haskell section in the nixpkgs manual. The content
presented here has been written from scratch, although some parts of it
take inspiration from the existing haskell4nix documentation.

It is by no means complete, the idea is mostly to get the ball rolling
in some way. Upcoming tasks are hinted at in the comments in the
documentation file.