doc: update references to nixfmt-rfc-style to treefmt

This completes https://github.com/NixOS/nixpkgs/issues/425583.

Rather than putting `nixfmt` everywhere, I opted to teach people to use
`treefmt` instead. This is more correct, as we have formatting rules for
non-nix files, and also may invoke `nixfmt` with non-default options.

pkgs/development/lisp-modules/import/main.lisp

@@ -49,7 +49,7 @@
           (truename "imported.nix")))
 
 (defun run-nix-formatter ()
-  (uiop:run-program '("nixfmt" "imported.nix")))
+  (uiop:run-program '("treefmt" "imported.nix")))
 
 (defun main ()
   (format t "~%")

pkgs/development/lisp-modules/shell.nix

@@ -1,10 +1,12 @@
 let
-  pkgs = import ../../../. { };
-  inherit (pkgs) mkShellNoCC sbcl nixfmt-rfc-style;
+  # Use CI-pinned (Hydra-cached) packages and formatter,
+  # rather than the local nixpkgs checkout.
+  inherit (import ../../../ci { }) pkgs fmt;
+  inherit (pkgs) mkShellNoCC sbcl;
 in
 mkShellNoCC {
   packages = [
-    nixfmt-rfc-style
+    fmt.pkg
     (sbcl.withPackages (
       ps:
       builtins.attrValues {