markdownlint/doc-build/md053.md

Links and images in Markdown can provide the link destination or image source
at the time of use or can use a label to reference a definition elsewhere in
the document. The latter reference format is convenient for keeping paragraph
text clutter-free and makes it easy to reuse the same URL in multiple places.

Because link and image reference definitions are located separately from
where they are used, there are two scenarios where a definition can be
unnecessary:

1. If a label is not referenced by any link or image in a document, that
   definition is unused and can be deleted.
2. If a label is defined multiple times in a document, the first definition is
   used and the others can be deleted.

This rule considers a reference definition to be used if any link or image
reference has the corresponding label. The "full", "collapsed", and "shortcut"
formats are all supported.

If there are reference definitions that are deliberately unreferenced, they can
be ignored by setting the `ignored_definitions` parameter to the list of strings
to ignore. The default value of this parameter ignores the following convention
for adding non-HTML comments to Markdown:

```markdown
[//]: # (This behaves like a comment)
```