Fix deadlinks
Merge pull request #591 from marocchino/fix-deadlink

Fix deadlinks about rubycentral
Fix typo
Remove unuse links
Move other implementation to about
Merge pull request #583 from marocchino/fix-typo-remove-unuse-link

Fix typo remove unuse link (en)
Extract installation (ko)
Merge pull request #582 from marocchino/ko-reorganize_download_and_installation

Extract installation steps to Installation page (ko)
Remove unmaintained (ko)
Merge pull request #592 from marocchino/ko-remove_unmaintained

Remove unmaintained (ko)