nixos: documentation: implement `documentation.dev.enable` option

I know that "devinfo" output does not currently exist, but so does "devman".
It is mentioned in the nixpkgs manual, but no derivation in nixpkgs actually uses it.

nixos/modules/misc/documentation.nix

@@ -50,6 +50,21 @@ let cfg = config.documentation; in
         '';
       };
 
+      dev.enable = mkOption {
+        type = types.bool;
+        default = false;
+        description = ''
+          Whether to install documentation targeted at developers.
+          <itemizedlist>
+          <listitem><para>This includes man pages targeted at developers if <option>man.enable</option> is
+                    set (this also includes "devman" outputs).</para></listitem>
+          <listitem><para>This includes info pages targeted at developers if <option>info.enable</option>
+                    is set (this also includes "devinfo" outputs).</para></listitem>
+          <listitem><para>This includes other pages targeted at developers if <option>doc.enable</option>
+                    is set (this also includes "devdoc" outputs).</para></listitem>
+          </itemizedlist>
+        '';
+      };
 
     };
 
@@ -60,20 +75,20 @@ let cfg = config.documentation; in
     (mkIf cfg.man.enable {
       environment.systemPackages = [ pkgs.man-db ];
       environment.pathsToLink = [ "/share/man" ];
-      environment.extraOutputsToInstall = [ "man" ];
+      environment.extraOutputsToInstall = [ "man" ] ++ optional cfg.dev.enable [ "devman" ];
     })
 
     (mkIf cfg.info.enable {
       environment.systemPackages = [ pkgs.texinfoInteractive ];
       environment.pathsToLink = [ "/share/info" ];
-      environment.extraOutputsToInstall = [ "info" ];
+      environment.extraOutputsToInstall = [ "info" ] ++ optional cfg.dev.enable [ "devinfo" ];
     })
 
     (mkIf cfg.doc.enable {
       # TODO(@oxij): put it here and remove from profiles?
       # environment.systemPackages = [ pkgs.w3m ]; # w3m-nox?
       environment.pathsToLink = [ "/share/doc" ];
-      environment.extraOutputsToInstall = [ "doc" ];
+      environment.extraOutputsToInstall = [ "doc" ] ++ optional cfg.dev.enable [ "devdoc" ];
     })
 
   ]);