fish: install docs to $out/share/doc/fish

Otherwise, it ended up going to $out/share/doc, which would pollute this
shared directory, as well as clash with other similarly misbehaving
packages.

pkgs/shells/fish/default.nix

@@ -125,6 +125,10 @@ let
       pcre2
     ];
 
+    cmakeFlags = [
+      "-DCMAKE_INSTALL_DOCDIR=${placeholder "out"}/share/doc/fish"
+    ];
+
     preConfigure = ''
       patchShebangs ./build_tools/git_version_gen.sh
     '';