Add this function to your Apps Script project and feed it an ID to a Google Doc.
If you need this on a lot of Docs, you may want to make a new Apps Script project to use as a library. Simply copy and paste this code to the new project, go to the script that's tied to your Doc, add your new Library (the script ID), and call var html = *NameOfYourLibraryProject*.getContent(DocumentApp.getActiveDocument().getId());