Doc: Updated logos on the landing page

Updated Qt and Qt Creator logos on the landing page, and removed
some obsolete information.

Task-number: QTBUG-41856
Change-Id: I6e4ff988c1611dd75ec8b040b506271821e0266f
Reviewed-by: Diana de Sousa <diana.desousa@digia.com>
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>