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
Fix /ja/documentation link caption
Fix sentence in  /ja/downloads 'Ways of installing Ruby' section
Fix /ja/installation/index.md
Fix ja/installation/index.md Homebrew section
Fix rbenv link caption /ja/installation/index.md
Unify "Linux, UNIX" to "Linux/UNIX"
Merge pull request #586 from bash0C7/transrate_instllation_article_ja

Add ja/installation & Fix parent pages