Source code, of course, because somebody may want to add features to the program. Examples of usage to supply a working basic configuration file and document it heavily. This list can be extended further. But in general a well-documented program will cover at least these, and it will do so in a readable fashion. Use your favourite text editor, as the formats I propose here can all be written using a text editor.
Style is worth mentioning as well.
Do not try to use poetry. The preferred language for documentation is plain English, as almost anyone who uses a computer is able to understand it. You can always add documentation in your native language as well. But keep in mind that not everyone speaks German or Russian. At least include English documentation for the most basic parts of your project.