Add "Installing Ruby" to documentation page (en)

Also add a link to the installation page.
Merge pull request #914 from stomar/update-documentation

Add "Installing Ruby" to documentation page (en)
Improve summary of the "Installers" section (en)

* remove duplicate information about ruby-build and ruby-install
  (they have dedicated sections by now)
* mention multiple installations
* add a note about RVM
Update Installation (ko)
Improve summary of the "Installers" section (ko)

* ref 24c27d2
Link to installation page from quickstart page (ko)

* ref c774ec1
Update Documentation (ko)
Add books link, Remove miss inputed lines (ko)
Translate 2014-11-25-ruby-2-2-0-preview2-released [vi]
Fix links and spacing (ko)
Merge pull request #916 from ruby-vietnam/translate-ruby-2-2-pr2

Translate 2014-11-25-ruby-2-2-0-preview2-released [vi]
Fix missing punctuation; rewrap (vi)
Merge pull request #913 from marocchino/ko-install

Update Installation (ko)
Remove links to pik from installation page

pik has been unmaintained for some time.
Remove links to pik from downloads page (de,en,it,ja,ko,pl)
Add uru to Ruby managers (en)
Add uru to Ruby managers (ko)
Specify packaged Ruby version on Arch Linux (en)
Merge pull request #917 from marocchino/ko-uru

Add uru to Ruby managers (ko)
add latest news post(ja): Rubyist Magazine 0049