- Add Doxygen-compliant comments to functions, enums, and state variables in `main.c`.
- This provides a foundation for automatically generating source code documentation.
- Remove the separate, now redundant, `firmware-manual.de.md` file.
- Create a new firmware manual (`firmware-manual.de.md`) to document the current features of the slave node.
- Add a linked table of contents to the new manual.
- Update the main `README.de.md` to link to the new firmware manual and the existing Modbus tool README.