Convert `summary_overview.html` to `summary_overview.md` once the file is ready

[dandhlee]

I'll be hard coding https://github.com/googleapis/python-bigquery-dataframes/blob/main/docs/templates/toc.yml#L7 to summary_overview.html as docfx tends to keep the file extension and does not convert them to what we want them to. While keeping it as summary_overview.html will continue to work, when the file is added we should rename the toc entry to summary_overview.md to ensure they're linked properly.