Small fixes on ruby-core page (it)

Add missing link, fix code indentation, add/remove linebreaks,
remove trailing whitespace.
Small fixes on user-groups page (it)

Fix indentation, remove trailing whitespace.
Fix heading levels on security page (ko)
Small fixes

Rewrap, fix YAML style, remove empty lines and trailing whitespace.
Fix news post author
Fix typo
Fix typo in link
Add rubyconferences.org to conferences page (en)
Small changes on conferences page (en)