How to write project documentation?

Question

Project management

At the moment we are developing a project, writing everything from scratch. The development is carried out by two programmers.
We plan to take 2-3 more people to the project, so I had a question - how to write documentation correctly so that a person who has just arrived does not pull hard on the programmers who are working, but read the docks and start working.
There may also be a situation that the programmer who wrote the project will leave tomorrow - but there is no information.
Please, tell me how you organize the process of writing docks. How much time is spent on it and who does it.

Answer 1

Yes, just look in the working project for example www.tinymce.com/wiki.php/Configuration
Only with the amendment that you write for developers and not for users.
What blocks, classes are responsible for what in short, where are the crutches that it is desirable to rewrite, explain where there are third-party modules, if any, explain the magic if there is one in the project.

Answer 2

I read the docks and started working, it is necessary to document the processes. From where to get the task, what are the rules for its development, delivery, testing. Rules for code ownership, commenting, refactoring, working with branches, rollouts, codestyle.