Implement tokens generation commands

dan/datasets/__init__.py

dan/datasets/entities/__init__.py

dan/datasets/entities/extract.py

dan/datasets/extract/__init__.py

dan/datasets/tokens/__init__.py

dan/datasets/tokens/generate.py

docs/ref/datasets/entities/extract.md

docs/ref/datasets/entities/index.md

docs/ref/datasets/tokens/generate.md

docs/ref/datasets/tokens/index.md

docs/usage/datasets/entities.md

docs/usage/datasets/extract.md

docs/usage/datasets/index.md

docs/usage/datasets/tokens.md

mkdocs.yml

tests/data/entities.yml

tests/data/export.sqlite

tests/data/tokens/end_tokens.yml

tests/data/tokens/no_end_tokens.yml

tests/test_entities.py