Update downloads, about, installation page (translations)

[stomar]

These pages have undergone major changes recently.

I suggest updating in two steps (like for de in 45129a3, 500ed05):

1. "Other Implementations" moved from downloads -> about

   • the first paragraph has been changed (also again later)
   • named link references have been introduced

2. Installation instructions moved from downloads -> documentation/installation

   • with major changes on both pages
   • new section on documentation, and a changed link on quickstart

Updated 2014-12-09

Please do not merge half finished updates, preferably use two clean commits.

[meisyal]

It seems about page (id) is outdated. I will work for it.