Update japanese docs to v4

I reflected the document change of v4 to Japanese pages.
Because some of the Japanese translation is not a good representation,
these will be fixed later

For all pages

* Add missing "/" to some links
* mv "community.html.erb" "localizable/community.html.erb" to use i18n

In english pages:

* Add Frontmatter: "title"
* Change some line breaks
* Fix some small problems

In japanese pages:

* Update locale YAML
* Add new page
* Update exist page
* remove unnecessary files