Add SyN registration examples

[jhlegarreta]

There is some interest in the community to add SyN registration examples to the ITK registration examples.

Cross-referencing ITK discourse post and ITK repository issue 1614.

May be this could also be added as an example to the ITK SW Guide by adding an example in the ITK codebase.

[jhlegarreta]

@zhang-qiang-github you are more than welcome to contribute with an example.

Community members will be happy to guide and help you through the process !