If your document has many headings, it's a good idea to add a table of contents in case it will be accessed outside Google Docs, for instance, if you export it as a PDF or EPUB document. You can even ...

You can create a table of contents in Microsoft Word where every chapter title links to the appropriate chapter. Then you can keep these links live when you convert the document to a PDF.

Create a fresh page and add Table of Contents text line, formatted with your favorite header style. Move your cursor just to the left of "Table". Choose Insert > Bookmark.