Doc: Edited "Core Internals" overview.

-edited sentences and titles.

Change-Id: Ia3ab0f355408828d685b344097355444fc12482e
Reviewed-by: Geir Vattekar <geir.vattekar@digia.com>
Doc: Edited "Testing and Debugging" overview.

-added links to Qt Creator manual
-fixed flow of information and sections

Change-Id: Ie35eada39050677db5e12d915d70d6d6c1bb86b4
Reviewed-by: Geir Vattekar <geir.vattekar@digia.com>
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>