Use a dedicated command to build language model resources

dan/datasets/__init__.py

dan/datasets/download/__init__.py

dan/datasets/download/images.py

dan/datasets/download/utils.py

dan/datasets/language_model/__init__.py

dan/datasets/language_model/build.py

dan/datasets/language_model/utils.py

docs/get_started/development.md

docs/get_started/training.md

docs/ref/datasets/language_model/build.md

docs/ref/datasets/language_model/index.md

docs/ref/datasets/language_model/utils.md

docs/usage/datasets/download.md

docs/usage/datasets/index.md

docs/usage/datasets/language_model.md

docs/usage/train/language_model.md

mkdocs.yml

tests/test_download.py

tests/test_language_model.py