Move installation page into documentation section

* rename files
* update internal links
* add rewrite rules
Merge pull request #911 from stomar/move-installation-page

Move installation page into documentation section
Link to installation page from quickstart page (en)
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)
Update Installation (ko)