treewide: Mark Nix blocks in markdown as Nix

This should help us with highlighting and future formatting.

nixos/modules/services/editors/emacs.md

@@ -178,7 +178,7 @@ file {file}`configuration.nix` to make it contain:
 ::: {.example #module-services-emacs-configuration-nix}
 ### Custom Emacs in `configuration.nix`
 
-```
+```nix
 {
  environment.systemPackages = [
    # [...]
@@ -203,7 +203,7 @@ adding it to your {file}`~/.config/nixpkgs/config.nix` (see
 ::: {.example #module-services-emacs-config-nix}
 ### Custom Emacs in `~/.config/nixpkgs/config.nix`
 
-```
+```nix
 {
   packageOverrides = super: let self = super.pkgs; in {
     myemacs = import ./emacs.nix { pkgs = self; };
@@ -228,7 +228,7 @@ only use {command}`emacsclient`), you can change your file
 ::: {.example #ex-emacsGtk3Nix}
 ### Custom Emacs build
 
-```
+```nix
 { pkgs ? import <nixpkgs> {} }:
 let
   myEmacs = (pkgs.emacs.override {
@@ -262,7 +262,7 @@ with the user's login session.
 
 To install and enable the {command}`systemd` user service for Emacs
 daemon, add the following to your {file}`configuration.nix`:
-```
+```nix
 services.emacs.enable = true;
 ```
 
@@ -323,7 +323,7 @@ In general, {command}`systemd` user services are globally enabled
 by symlinks in {file}`/etc/systemd/user`. In the case where
 Emacs daemon is not wanted for all users, it is possible to install the
 service but not globally enable it:
-```
+```nix
 services.emacs.enable = false;
 services.emacs.install = true;
 ```