Add docstrings on all modules

arkindex_worker/cache.py

arkindex_worker/git.py

arkindex_worker/image.py

arkindex_worker/models.py

arkindex_worker/reporting.py

arkindex_worker/utils.py

arkindex_worker/worker/__init__.py

arkindex_worker/worker/base.py

arkindex_worker/worker/classification.py

arkindex_worker/worker/element.py

arkindex_worker/worker/entity.py

arkindex_worker/worker/metadata.py

arkindex_worker/worker/transcription.py

arkindex_worker/worker/version.py