DOC: Update the ITK code style to current automatically enforced

Update the ITK code style to the current, automatically enforced style:
- Introduce the use of the `clang-format` style enforcing tool.
- Modify the related passages.
- Update the verbatim code snippets to reflect the new style.

Cross-referencing the ITK change commits:
https://github.com/InsightSoftwareConsortium/ITK/pull/1191/commits

Author: jhlegarreta