This reference is written in AsciiDoc and follows the directory conventions of the documentation site generator Antora.
A good place to start is modules/ROOT/pages/introduction.adoc.
Screenshots are generated on macOS as part of the Helgobox integration test.
After executing the tests via Extensions › Helgobox › Run integration test, they can be found at DOCUMENTS_DIR/realearn-screenshots and should then be copied to modules/ROOT/images/realearn/screenshots.