refactor: Reset counter in post build event

This change helps keeping stable ids
across rebuilds and live-reload.

src/markdown_exec/mkdocs_plugin.py

@@ -13,6 +13,7 @@
 
 from markdown_exec import formatter, formatters, validator
 from markdown_exec.logger import patch_loggers
+from markdown_exec.rendering import MarkdownConverter
 
 if TYPE_CHECKING:
  from jinja2 import Environment
@@ -70,3 +71,6 @@ def on_env(self, env: Environment, *, config: Config, files: Files) -> Environme
  write_file(css_content.encode("utf-8"), os.path.join(config["site_dir"], css_filename))
  config["extra_css"].insert(0, css_filename)
  return env
+
+ def on_post_build(self, *, config: Config) -> None: # noqa: ARG002,D102
+ MarkdownConverter.counter = 0