1) Write short, very short content
Even the shortest as possible, provided that it still delivers exact information. Indeed, users browse documentation when they look for solution or help. So, they are in a hurry to find it, and conversely they do not have time for reading, especially not for fun.
2) Provide screenshots, infographics, diagrams
or even mockups. There are plenty of studies showing that images retain attention and are far better memorized than plain text.
3) Make all its content searchable and provide a searchbar
Users barely read chapters or sections titles, but they first want to search for specific terms. So, make their search more convenient with a search function flexible and powerful enough to work with a large kind of queries.
4) Get some statistics
about which are the most read topics (of your documentation). Then, highlight them with a direct link on the homepage or other means of featuring. Users will thank you to make them earning time.
5) Invite users to send you feedback, question or comment
This is the most important, and you can do it by this simple way: on all pages, display a one-click button (or image, or link) that opens a form, where users enter address email and feedback. Only these two fields, this is quick and easy.