ci/treefmt: add markdown-code-runner

This was run as a test in `doc/tests/check-nix-code-blocks.nix` before,
but its DX can be improved: By including it in `treefmt` we get better
error reporting and auto-fixing, as well as running it on *all* markdown
files (including READMEs etc.) for free.

(cherry picked from commit a7b3b2584a)

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

@@ -1,32 +0,0 @@
-{
-  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

@@ -1,3 +0,0 @@
-[presets.nixfmt]
-language = "nix"
-command = ["nixfmt"]