Move installation page into documentation section

Currently, the `installation` page is not integrated into the site structure, it's not part of any of the categories present in the main navigation. This commit moves it into the documentation section, also updating all existing links (right now there are links only from the `downloads` page).

In a follow-up commit I will add a link to the instructions from the `documentation` page ("Getting started" with Ruby usually also implies installing it).

@postmodern @hsbt @chikamichi @makimoto I would appreciate a short feedback/approval.