Revert "darktable: Remove doc directory from output"

This reverts commit 730133e1.

multiple-outputs.sh has been fixed, so documentation is now
corrrrectly installed under $out/share/doc/darktable.

Fixes: https://github.com/NixOS/nixpkgs/pull/72160
Fixes: https://github.com/NixOS/nixpkgs/pull/83248