For example, the Python library reference has over 1,000 pages. To read all pages is very tough. You can not keep your motivation. However, the table of contents has only 6 pages. There are only 34 lines of section headings. The motivation to read them is easy to keep. So you should read them first. Then you grasp the overall picture of the 1,000 pages document. Although it is very rough.
In books and large-scale materials, it is common practice to place a table of contents at the beginning. It is to make it easy to grasp the overall picture. Many speed-reading techniques encourage you to focus on the table of contents and section headings. Start roughly and gradually detailed. *1
*1 Regarding how to read books, I explain them in detail in (4.3.3) Attention to headlines.