Doc: Update Qt Enterprise information and supported platforms

Mention the availability of Qt Enterprise, Mobile Edition
on index page and whenever linking to open-source downloads
page on qt-project.org.

Also, update the following:
    - Reference platforms (OS X, Windows versions, mobile platforms)
    - Embedded platforms (Emb. Android, Qt Enterprise Embedded)
    - Divide Mobile platforms: Digia-supported & the rest
    - Mention other mobile platforms (Sailfish, Tizen, WinRT)

Task-number: QTBUG-33360
Change-Id: I36d74ca36699e24737b46af7a4a6f2736ada64af
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Sami Makkonen <sami.makkonen@digia.com>