Doc: Replaced the HTTPS links with HTTP

The HTTPS links fail in Qt Assistant on Windows as the
qt installation package includes qt libraries that are
built without SSL support for legal reasons.

Task-number: QTBUG-31073
Change-Id: I86909abadb1e8164749d924cc53ee05aa57f8f31
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Karsten Heimrich <karsten.heimrich@digia.com>