Don't write sentences. Write bullet points. Arrange those bullet points into groups. That becomes your table of contents. Continue to add more bullet points over time, and rearrange them as necessary. Eventually, you'll get the picture of what the documentation should look like. Then you can convert the bullet points into prose.

The reason you do it this way is that it's way easier to write bullets that it is to write sentences. It allows you to focus on substance instead of form. It's analogous to typing into a plain text editor and not worrying about formatting. Formatting, and form, do matter, a lot. But you can add them later.