Kirill,
I have an essay at my web site called A Mathematica Style that among other things gives an overview of using Workbench to create documentation. Documentation would usually be part of a larger entity called a Mathematica Application, which might include various combinations of notebooks, packages, style sheets. palettes and documentation. The essay discusses how to lay this all out. Mathematica Applications are quite a standard part of Mathematica and the Wolfram Language and a good way to organize work.
Workbench comes with Premier service but there may be other ways to obtain it. You would have to contact Support.
Recent versions of Mathematica, V10+, require Workbench 3. This is not even in formal beta testing, but it usually can be obtained by contacting Wolfram. I use it constantly for documentation and it works perfectly well. I don't know what Wolfram's long term plans are for documentation. I just cross my fingers and hope that whatever it is, it will maintain compatibility with present paclet documentation.
