The KDE project uses docbook for generating your project handbook (i.e. the user manual). The user manual is available through your application menubar by choosing Help->YourApplication Handbook when your KDE GUI application is running. After building your project, the user manual is displayed in KDE KHelpCenter. It should explain to the user how your application works, what are the main features and how to configure it. It should also explain the advanced features if any.

All KDE based templates in KDevelop have a doc subdir which contains a index.docbook template in the en folder to get you started in writing the user manual. You should edit this index.docbook in KDevelop and start changing personal details such as name, email, etc.. Look at the comments in this file and try following these indications to start your application documentation. Rebuild your project to see the changes in KHelpCenter.

You can find more about the docbook syntax on the KDE documentation website.