Doc: Replace 'Learn More' subsections with bolded text

Getting Started Programming with Qt Widgets has multiple subsections
titled 'Learn More'. They appear in TOC but do link to correct
sections.

This change fixes the issue by replacing these sections with bolded
text so they are not listed in TOC.

Task-number: QTBUG-32110
Change-Id: I4fbabcc18dc15737f5a155c742a82cefe57ba17d
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>