docs: Create a markdown-it plugin

flake-modules/dev/default.nix

@@ -52,6 +52,9 @@
             "**.scm"
             "**.svg"
             "**/man/*.5"
+            # Those files are generated by pytest-regression, which then `diff`s them.
+            # Formatting them will make the tests fail.
+            "docs/gfm-alerts-to-admonitions/tests/**/*.yml"
           ];
           formatter.ruff-format.options = [ "--isolated" ];
         };