Some content files might be leftovers or should be excluded in `_config.yml`

[schnaitter]

When building the book locally I get messages for documents that are not "linked in the toctree". Some of them I exclude/will exclude in PR #225.

The following should either be deleted or excluded in the _config.yml to more easily spot errors when building the book.

checking consistency... $PROJECTS/Text-Fallstudie-3/corpus_collection/corpus-collection_metadata-extraction.ipynb: WARNING: document isn't included in any toctree
$PROJECTS/Text-Fallstudie-3/corpus_processing/corpus-processing_analysis.md: WARNING: document isn't included in any toctree
$PROJECTS/Text-Fallstudie-3/präambel/lernziele.md: WARNING: document isn't included in any toctree
$PROJECTS/Text-Fallstudie-3/präambel/technische_voraussetzungen.md: WARNING: document isn't included in any toctree
$PROJECTS/Text-Fallstudie-3/präambel/toc.md: WARNING: document isn't included in any toctree
$PROJECTS/Text-Fallstudie-3/präambel/vorkenntnisse.md: WARNING: document isn't included in any toctree
done

[schnaitter]

#230 should be fixed before deleting präambel directory