Doc: Fix example pages for Qt Tools

Use the \group and \ingroup commands to produce Qt 5 style
example pages. Unify the file structure for example docs.
Add example page for Qt Help and move Context Sensitive Help
example to it (from the Qt Assistant Examples page).

Change-Id: I687b18a134037588a5a536fe2353c18dfd44b1d7
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: David Schulz <david.schulz@digia.com>