nixos/documentation: Link Devhelp files

While Devhelp now supports finding API documentation in `/share/doc` and the new `gi-docgen` toolchain places it there, older projects using gtk-doc (e.g. GLib) still install it to `/share/gtk-doc` and/or `/share/devhelp/books`.

For people using the major DEs this is not problem since they have `/share` in `pathsToLink` but we want to be explicit and make it work for everyone.

nixos/modules/misc/documentation.nix

@@ -350,7 +350,13 @@ in
     })
 
     (mkIf cfg.doc.enable {
-      environment.pathsToLink = [ "/share/doc" ];
+      environment.pathsToLink = [
+        "/share/doc"
+
+        # Legacy paths used by gtk-doc & adjacent tools.
+        "/share/gtk-doc"
+        "/share/devhelp"
+      ];
       environment.extraOutputsToInstall = [ "doc" ] ++ optional cfg.dev.enable "devdoc";
     })