{nixpkgs-manual, nixos-manual}: add new test `check-nix-code-blocks`

doc/doc-support/package.nix

···
   nixos-render-docs-redirects,
   writeShellScriptBin,
   nixpkgs ? { },
+  markdown-code-runner,
 }:
 
 stdenvNoCC.mkDerivation (
···
           packages = [
             devmode'
             nixos-render-docs-redirects'
+            markdown-code-runner
           ];
         };
 
-      tests.manpage-urls = callPackage ../tests/manpage-urls.nix { };
+      tests = {
+        manpage-urls = callPackage ../tests/manpage-urls.nix { };
+        check-nix-code-blocks = callPackage ../tests/check-nix-code-blocks.nix { };
+      };
     };
   }
 )

doc/tests/check-nix-code-blocks.nix

···
+{
+  runCommand,
+  markdown-code-runner,
+  nixfmt-rfc-style,
+}:
+
+runCommand "manual_check-nix-code-blocks"
+  {
+    nativeBuildInputs = [
+      markdown-code-runner
+      nixfmt-rfc-style
+    ];
+  }
+  ''
+    set +e
+
+    mdcr --check --config ${./mdcr-config.toml} ${./..}
+
+    if [ $? -ne 0 ]; then
+      cat <<EOF
+    Error: `mdcr` command failed. Please make sure the Nix code snippets in Markdown files are correctly formatted.
+
+    Run this command from the Nixpkgs repository root for automatic formatting:
+
+        mdcr --log debug --config ${toString ./..}/tests/mdcr-config.toml ${toString ./..}
+
+    EOF
+      exit 1
+    fi
+
+    touch "$out"
+  ''

doc/tests/mdcr-config.toml

···
+[presets.nixfmt]
+language = "nix"
+command = ["nixfmt"]