GitLab

Should be reviewed by a native speaker.

Also:
- clean unused files in pics/
- (re)add some minor german spell checking
- remove PDFs from gitignore, to allow PDF files in pics/

The last item has the consequence that the PDF file of document
generated by pdflatex gets added to the history at each commit,
increasing the size of .git invain. For the moment it's not clear how we
could get rid of this disadvantage.

I forgot capital letters before.

The line
\usepackage[applemac]{inputenc}
is added and commented out because I don't know exactly what is needed
on a macOS, maybe all works fine as it is there as well, as it should
do on Linux.

with placeholder file for C++ and to-be-revised material for ROOT.