Zh tw/download and associated updates

Corresponding English page are 
https://ruby-lang.org/en/downloads/ and https://ruby-lang.org/en/documentation/installation/.

This pull request updates https://ruby-lang.org/zh_tw/downloads/ and https://ruby-lang.org/zh_tw/documentation/installation/ page.

Get preview on local:

```
$ git clone git@github.com:ruby/www.ruby-lang.org.git
$ cd www.ruby-lang.org
$ bundle
$ rake preivew
```

The `rake preivew` could take up to 10 minutes (depends on how fast your machine), please be patient :wink: !

`zh_tw` preview site will be available on http://localhost:4000/zh_tw.
原文的 "Certain members in the Ruby community feel very strongly that you should never use a package manager to install Ruby and that you should __use tools instead__."  直接寫 RVM 好像會有點誤導？改成 "而應該使用其它的工具" 呢？

"Ruby 版本老舊~~，要使用~~ __。如果你想要使用__ 最新版的 Ruby..."

"~~確定你使用了正確的套件名稱~~ __確認你安裝了正確名稱的套件__"