Fix QtQml module docs following doc restructure

The QtQml module docs need to be extensively revised following the QML doc restructure.

When fixing a doc page, ensure that:

• All section headers use title case
• The page uses the new recommended terminology for Type, Component, Document etc.
• Each page title is an appropriate and succinct description of the page content. The title should inform the user of the value that the page gives to the user so s/he knows what to expect from the page before clicking on any title link to that page
• Broken links are fixed