Doc: Update names of available Qt licenses

And link to the download page for purchasing and details on each
license type.

Task-number: QTBUG-42550
Change-Id: I4bc565a386e9bbfbff91f9a2e636ea6ad3a5ffb9
Reviewed-by: Sami Makkonen <sami.makkonen@digia.com>
Reviewed-by: Tuukka Turunen <tuukka.turunen@digia.com>
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>
Reviewed-by: Venugopal Shivashankar <venugopal.shivashankar@digia.com>