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
Translate the rest part of installation into ja
Fix mistranslation
Fix mistranslation ja/installation/index.md
Translate `Compiling Ruby — Source code` and `Mirror sites` section