make pyodide plugin sphinx-markdown-builder compatible

[MarcSkovMadsen]

In panel-extensions/panel-material-ui#366 I'm trying to add sphinx-llm. It almost works. The only remaining issue is that pyodide output gets added to the .html.md files created. See jacobtomlinson/sphinx-llm#29 for detail.

Please make the nbsite pyodide plugin sphinx-markdown-builder compatible. It should not add the pre pyodide output when building markdown output.

Thanks.