rustc: Install docs in a separator output

This reduces rustc's size from 359 MiB to 186 MiB.

Maybe we should disable doc generation altogether, since it's very
slow and people who need docs will JFGI anyway. But for now I've left
it.

pkgs/development/compilers/rustc/generic.nix

@@ -136,6 +136,8 @@ stdenv.mkDerivation {
 
   enableParallelBuilding = true;
 
+  outputs = [ "out" "doc" ];
+
   preCheck = "export TZDIR=${tzdata}/share/zoneinfo";
 
   doCheck = true;