How to develop user manuals?

For you not to worry so much about formatting, and get focus on writing the contents of the documentation, I suggest these tools:

• Using LaTex you take advantage of an existing formatting template and generate a high-quality PDF. An example of excellent documentation done at LaTex: manual / manual source
• Using the MarkDown syntax you can create HTML documentation.
• Git will help you keep track of versions of your documentation.
• A good text editor, such as Sublime Text, which has plugins to help you use the tools above.

Softwares that help make user manuals are common to help you produce any written content. It will mainly be a word processor and eventually others that might help illustrate the text. There is nothing to help write the user manual. Eventually you can use some software to put it in a specific format that can be read by some standard software. But they will not make the work of months turn into days. It will only do a finish after the manual is ready.

The other softwares described in the question help document API. So we're not talking about a user manual. They help a bit, but it's basically formatting, API documentation should be done carefully and this will be responsible for almost any time taken.

I can only recommend testing everyone and come to their conclusion which helps you the best. Random people on the internet can not tell you what's good for you.

I believe that other aspects do not fit here in SOpt.

You can use Text Editor templates to format your document.

If you master, you can use HTML with a CSS Framework and develop a Web manual.

You can develop a manual in Latex (also interesting to master). The finish is good and is easy to do once you master the language.

You can create video tutorials in the system.

You can use a presentation editor with a template to leave everything as default. It can even combine with videos.

You can use desktop publishing software like Scribus to help you finish.