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.

ci/default.nix

@@ -75,6 +75,23 @@ let
           includes = [ "*" ];
           priority = 1;
         };
+
+        # TODO: Upstream this into treefmt-nix eventually:
+        #   https://github.com/numtide/treefmt-nix/issues/387
+        settings.formatter.markdown-code-runner = {
+          command = pkgs.lib.getExe pkgs.markdown-code-runner;
+          options =
+            let
+              config = pkgs.writers.writeTOML "markdown-code-runner-config" {
+                presets.nixfmt = {
+                  language = "nix";
+                  command = [ (pkgs.lib.getExe pkgs.nixfmt) ];
+                };
+              };
+            in
+            [ "--config=${config}" ];
+          includes = [ "*.md" ];
+        };
       };
       fs = pkgs.lib.fileset;
       nixFilesSrc = fs.toSource {