Doc: Edited Getting Started page.

-Getting Started page is the same as the "installation" page.
-removed older installer documentation
-changed wording of the download information
-removed unnecessary information

Task-number: QTBUG-33360
Change-Id: I07855bd306fbfe369f7a859619d5bf274bbf5972
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>