Merge repo 'ampel-documentation' into 'ampel-firmware'
Keeping them as two separate repositories does not make
much sense, IMHO. Users that do not want to touch the
code and simply need a manual will use the "released" documentation
in https://www.hft-stuttgart.de/fileadmin/Dateien/Forschung/Downloads/Bauanleitung-CO2-Ampel.pdf, anyway.
For other cases where some code change in ampel-firmware
involves some
change of functionality that has to be documented, it does make more sense
to have the repos together and have the corresponding changes in one MR or
even in one commit.