Check out Convert Google Docs content to Markdown or HTML using the gd2md-html add-on by . Here is an excerpt:
gd2md-html is a Google Docs add-on that will convert your Google Doc content into either Markdown or HTML. This tool provides a much-needed converter that enables you to use Google Docs as a platform for content development without manually reformatting the content when you’re ready to publish…
The full article is available here.