reference/index更新、reference/twig_referenceとreference/configuration/kernel追加,cookbook/configuration/override_dir_structure追加,reference/forms/twig_reference翻訳

- [x] reference/configuration/mongodb.rst削除
- [x] reference/index.rst更新
- [x] reference/map.rst.inc更新
- [x] reference/mode.rst削除
- [x] reference/YAML.rst削除
- [x] reference/configuration/kernel.rst追加
- [x] reference/twig_reference.rst追加
- [x] cookbook/configuration/override_dir_structure.rst追加
- [x] cookbook/configuration/index.rst追加
- [x] reference/forms/twig_reference.rst翻訳（未翻訳）