Make download links clearly
Extract installation steps to Installation page
Move alternative impls to about page
Merge pull request #576 from sorah/reorganize_download_and_installation

Extract installation steps to Installation page
Extract a page about mirror sites
Add `Ruby Referense manual Ruby 2.1.0 version` link (ja)
Merge pull request #585 from bash0C7/link_ruby_reference_manual_ruby_2.1.0_ja

Add `Ruby Referense manual Ruby 2.1.0 version` link (ja)
Fix typo (s/confortable/comfortable/g)
Merge pull request #584 from makimoto/extract-mirrors

Extract a page about mirror sites
Fix typo