Google Docs is the top online document editor, hands down. If you're working on document, you can quickly create an outline in Google Docs.

You can create & add a document outline in Google Docs using this guide. Difference between Table of contents & Document Outline explained.

Without adding headings, you can't create an outline or table of contents in Google Docs. Headings let you easily jump through different sections or move straight to the summary in long documents.