Google: Table of Contents for Google Docs

A table of contents makes it easy for you, your collaborators, and viewers to quickly navigate to a section of your document. Each item in your table of contents links to the titled sections of your document that use the heading styles. Here's how to add one to your document:

1. Go to the Format menu and select Paragraph styles to add headings to sections of your document. There are six different heading sizes to choose from.
2. Place your cursor where you'd like to insert the table of contents.
3. Go to the Insert menu, and select Table of contents.
4. If you need to move the table of contents, select it as you would select text and either move it with your cursor or cut it and paste it.
5. You can continue to add headings to your document or change current headings. However, if you'd like a change to become part of the table of contents, you need to click first the table and then the Refresh button. 

Note: If you want to change the text of the table of contents, edit the headings in the document body rather than in the table of contents. Once you update the table of contents by clicking the Refresh button, any manual edits you made in the table of contents will be cleared.


Reference:
Table of contents in documents
https://support.google.com/docs/answer/106342?hl=en