Doc: Fixed link to the "Getting Started with Qt" from platforms' pages.
Doc: Fixed link to the "Getting Started with Qt" from platforms' pages.

-Placed the link higher because the page discusses downloading and
installation.

Change-Id: I67449549cb419bc845c691f06a4c45fbb2ddee7d
Reviewed-by: Martin Smith <a8cc6484db2755056ccf1c457da8ca771f895dea@digia.com>

kobolabs/qtdoc,kobolabs/qtdoc,kobolabs/qtdoc,kobolabs/qtdoc