Doc: Add information about the installers from Qt Project.

The installers are summarized in a table. The platform pages have links
to the installer page.

Change-Id: Iee2f98556f498118cc4055bf7fc8e6d346db8d69
Reviewed-by: Jani Heikkinen <jani.heikkinen@digia.com>
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>
Reviewed-by: Sergio Ahumada <sergio.ahumada@digia.com>