Improve the class listing in "Subclassed by"

[jhlegarreta]

The list of classes that appears in the "Subclassed by" section of the examples contains a variable number of elements, depending on whether the class or classes demonstrated is/are a superclass of other classes. In some cases, this list can be very long, e.g. https://itk.org/ITKExamples/src/Core/Common/FilterImage/Documentation.html, and the readability is low.

If possible, some changes might improve the readability and discoverability of the subclasses:

• Avoid the template parameter list, or apply the nice formatting applied to the classes in the "Classes demonstrated" section.
• Make the classes point to their corresponding Doxygen documentation. If not possible, maybe to examples demonstrating the classes if existing.
• If the above are not possible, maybe we can change the whole section for a link to the class hierarchy that is displayed in the Doxygen pages.
• By default, hide the list and just print "Subclassed by" which would expand when clicking.