fix(cli): improve docs error messages around references to missing mdx files

[dsinghvi]

Description

Before:

After:

Changes Made

• Use fdr-sdk to remove false positives on missing markdown files when resolving slugs
• Adds a fern check rule that is keyed based on each mdx file and prints which file is referenced and not included in docs.yml

Testing

• Unit tests added/updated
• Manual testing completed

[github-actions]

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