Encourage users to install the ruby-full meta-package.

* This will also install ruby-dev, which is needed for compiling any C
  extensions.
Emphasis installing ruby from the package manager.

* Most users just need a Ruby so they can run sass or jekyll.
  Whereas RVM/rbenv are more suited for developers and power-users.
Organize in-page links by installation method than by platform/tool.

* Combined the Package Management Systems sub-list into the top list.
Split the "Third party tools" section into "Installers" and "Managers".
Added ruby-build and ruby-install to the Installers section.
Added chruby to the Managers section.
Put Installers before Managers.
Add a summary to the Installers section.
Link to the chruby section.
Added Ruby 2.1.0 Release Post (tr)
Updated some text
Added the confoo call for paper
Added the introduction of the conference
Removed duplicate line
Merge pull request #816 from Sdogruyol/master

Added Ruby 2.1.0 Release Post (tr)
Merge pull request #818 from ylarrivee/master

Confoo call for paper
Translate confoo 2015 CFP (zh_tw).
ConFoo 2015 (fr)
CooFoo 2015 cfp (tr)
Update Ruby User’s Guide text of documentation (zh_cn)
Merge pull request #824 from wuleicanada/patch-2

Update Ruby User’s Guide text of documentation (zh_cn)