Add ja/installation & Fix parent pages

- Add link `ja/downloads` to `ja/installation`
 - and translate `Ways of installing Ruby` section
- Add link `ja/documentation` to `ja/instllation`
- Fix link `ja/documentation` to `ja/downloads`
- (Under translation) Translate `en/installtion/index.md` into `ja/installation/index.md`
 - Probably I complete translation of this page by the end of this week.