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.

doc/languages-frameworks/python.section.md

@@ -2136,7 +2136,7 @@ The following rules are desired to be respected:
 * `pythonImportsCheck` is set. This is still a good smoke test even if `pytestCheckHook` is set.
 * `meta.platforms` takes the default value in many cases.
   It does not need to be set explicitly unless the package requires a specific platform.
-* The file is formatted with `nixfmt-rfc-style`.
+* The file is formatted correctly (e.g., `nix-shell --run treefmt`).
 * Commit names of Python libraries must reflect that they are Python
   libraries (e.g. `python3Packages.numpy: 1.11 -> 1.12` rather than `numpy: 1.11 -> 1.12`).
   See also [`pkgs/README.md`](https://github.com/NixOS/nixpkgs/blob/master/pkgs/README.md#commit-conventions).

pkgs/applications/editors/vscode/extensions/README.md

@@ -7,7 +7,7 @@
 
 * When adding a new extension, place its definition in a `default.nix` file in a directory with the extension's ID (e.g. `publisher.extension-name/default.nix`) and refer to it in `./default.nix`, e.g. `publisher.extension-name = callPackage ./publisher.extension-name { };`.
 
-* Currently `nixfmt-rfc-style` formatter is being used to format the VSCode extensions.
+* Use `nix-shell --run treefmt` to format the VSCode extensions.
 
 * Respect `alphabetical order` whenever adding extensions. If out of order, please kindly open a PR re-establishing the order.
 

pkgs/by-name/ni/nixfmt-tree/package.nix

@@ -95,7 +95,7 @@ treefmtWithConfig.overrideAttrs {
       You can achieve similar results by manually configuring `treefmt`:
       ```nix
       pkgs.treefmt.withConfig {
-        runtimeInputs = [ pkgs.nixfmt-rfc-style ];
+        runtimeInputs = [ pkgs.nixfmt ];
 
         settings = {
           # Log level for files treefmt won't format

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 {