Add broken link checker

### Description

The ITK documentation (whether it is Markdown files or class documentation) frequently contains links to diverse websites. Some of these links may be broken. A tool (a pre-commit hook, and/or a GHA workflow) to check for broken links would prevent from including broken links, or would point out which links are broken across the code base.

### Expected information

Broken links should not exist.

### Actual information

Broken links may exist in the ITK documentation.

### Versions

`master`

### Additional Information

This probably applies to the ITK SWG, and maybe the ITK Sphinx Examples as well.