Add a description about ruby toolbox to libraries [ja]

In this pull req, I add a description about The Ruby Toolbox to [/ja/libraries](https://www.ruby-lang.org/ja/libraries).
It is translated from [the English version](https://www.ruby-lang.org/en/libraries).

Additionally, I replace numeric references for links with named ones to make it easy to modify the entry in future.
(eg. `[1]: https://rubygems.org/` to `[rubygems]: https://rubygems.org`)