Add audit mode for un-utilized/linked content

[StephenHinck]

A couple scenarios come to mind:

• *.mdx files which do not appear within docs.json
• Image files which do not appear within any *.mdx files

As we grow over time, and in particular, as we need to manually update our API documentation due to wanting to include custom image slugs, we need a way to validate whether all expected content exists within the structure. When we build the docs, mintlify already tells us about bad/missing links, but what about the opposit?