There's docbook, but to be fair, docbook excels at book or manual style documentation. It does do website documentation, but its websites tend to not be as heavily interlinked as javadoc. This is an example of a docbook document rendered to a website.
Docbook excels when you have one "source document" that doesn't have a presentation in the real world, and a few "rendered documents" that all must contain correct, up to date information built from the source document.
It is hard to understand what you are looking for, mainly because it seems that you are not exactly sure yourself. Javadoc is documentation that was purpose designed to document Java code, so a "javadoc like documentation without source code" is a hard idea to follow. Perhaps if you sat down and thought about the main sections of documentation, you'd find an outline (which might heavily include the navigation you want to use). Then it would be much easier to fill in the blanks, because you have blanks.
You can use the javadoc engine to generate documentation. It is very configurable; and supports two kinds of plugins. One to add additional "tags" and one to add / override additional "layouts". However, these tags and layouts operate on an API that tends to be Java source code oriented, such that a "javadoc comment before a class" calls a method that effectively handles the javadoc for this class. As such, without Java source code (or something very like it) you have the wrong underlying API (unless you can bend it to your needs).