[Feature] Add Registry (#11)

docs/en/api.rst

docs/en/index.rst

docs/zh_cn/api.rst

docs/zh_cn/index.rst

mmengine/__init__.py

mmengine/registry/__init__.py

mmengine/registry/registry.py

mmengine/registry/root.py

mmengine/utils/misc.py

tests/test_registry/test_registry.py