JBIDE-17122 - Website: download section improvements

Fixed link to latest stable version of JBT and DevStudio on /downloads (older version was retained)
Set "archived: true" on DevStudio 7.1.0.GA (it does not appear on side nav menu of individual download pages)
Added an "Installer" section similar to the "update site Zips" one with links to download standalone or
standalone with EAP (when configured in products.yml)
Fixed "unknown" file type (some URLs ended with "/download", which was not an expected file extension)
Added a log message when an unknown file type is detected, for easier fixing.