Doc: Add links to the index page

Create links for the topics mentioned on the index page for
Qt Framework, Qt Creator, Tools and Toolchains, and Licenses
Used in Qt.

Task-number: QTBUG-42552
Change-Id: Ia493e88fcb93319cacd5997288ab0f3b832d064c
Reviewed-by: Tuukka Turunen <tuukka.turunen@digia.com>
Reviewed-by: Venugopal Shivashankar <venugopal.shivashankar@digia.com>